according to the known principles of optics. The fame experiments he made with globes of feveral diameters; but he found, that, whereas the shadows of the cylinders did not disappear but at the distance of 41 of their diameters, thofe of the globes were not vifible beyond 15 of their diameters; which he thought was owing to the light being inflected on every fide of a globe, and confequently in fuch a quantity as to difperfe the thadows fooner than in the cafe of the cylinders. In all these cafes, the penumbra occafioned by the inflected light began to be vifible at a less diftance from the body in the ftronger light of the fun than in a weaker, on account of the greater quantity of rays inflected in thofe circumstances.

142. M. MARALDI, being fenfible that the above mentioned phenomena of the fhadows were caused by inflected light, was induced to give more particular attention to this remarkable property, and to repeat the experiments of Grimal di and Sir Ifaac Newton in a darkened room. In doing this, he observed, that, befides the enlarged fhadow of a hair, a fine needle, &c. the bright gleam of light that bordered it, and the 3 coloured rings next to this enlightened part, when the fhadow was at a confiderable diftance from the hair, the dark central shadow was divided in the middle by a mixture of light; and that it was not of the fame denfity, except when it was very near the hair. This new appearance is exactly fimilar to what he had obferved with respect to the shadows in the open day-light above-mentioned..

143. Having placed a briftle in the rays of the fun, admitted into a dark chamber by a small hole, at the diftance of 9 feet from the hole, it made a fhadow, which, being received at 5 or 6 feet from the object, he obferved to confift of feveral streaks of light and fhade. The middle part was a faint penumbra, bordered by a darker fhadow, and after that by a narrower penumbra; next to which was a light ftreak broader than the dark part, and next to the ftreak of light, the red, violet, and blue colours were feen as in the fhadow of the hair. He alfo placed, in the fame rays, feve ral needles of different fizes; but the appearances were fo exceedingly various and fingular, that he does not recite them. He expofed in the rays of the fun admitted by a small hole into a dark chamber, a plate two inches long, and a little more than half a line broad. This plate being fixed perpendicularly to the rays, at the distance of nine feet from the hole, a faint light was feen uniformly difperfed over the fhadow, when it was received perpendicularly to it, and very near. The fhadow of the fame plate being received at the diftance of 24 feet, was divided into four very narrow black ftreaks, feparated by small lighter intervals equal to them. The boundaries of this fhadow on each fide had a penumbra, which was terminated by a very strong light, next to which were the coloured freaks of red, violet, and blue, as before. This is reprefented in Plate 250, fig. 4. 144. The fhadow of the fame plate, at 4 feet distance from it, was divided into two black ftreaks only, the two outermoft having difappeared, as in fig. 5; but these two black streaks which remain. ed were broader than before, and separated by a lighter fhade, twice as broad as one of the former

black streaks, when the fhadow was taken at 2 feet. This penumbra in the middle had a tinge of red. After the two black streaks there appeared a pretty ftrong penumbra, terminated by the two freaks of light, which were now broad and fplendid, after which followed the coloured freaks. A 2d plate, 2 inches long and a line broad, being placed like the former, 14 feet from the hole by which the rays of the fun were admitted, its fhadow being received perpendicularly very near the plate, was illuminated by a faint light, equally dif persed, as in the preceding plate. But being received at 13 feet from the plate, fix fmall black ftreaks began to be vifible, as in fig. 6. At 17 feet from the plate, the black streaks were broad. er, more diftinét, and more feparated from the ftreaks that were lefs dark. At 42 feet from the plate, only two black ftreaks were feen in the middle of the penumbra, as in fig. 7. This middie penumbra between the two black streaks was tinged with red. Next to the black streaks there always appeared the streaks of light, which were broad, and the coloured freaks next to them. Receiving the fhadow of the fame plate at the dif tance of 72 feet, the appearances were the fame as in the former fituation, except that the two black ftreaks were broader, and the interval between them, occupied by the penumbra, was broader alfo, and tinged with a deeper red.

145. In the fame rays of the fun he placed different plates, larger than the former, one of them a line and a half, another two lines, another 3 lines broad, &c. but, receiving their fhadows upon paper, he could not perceive in them thofe ftreaks of faint light which he had obferved in the fhadows of the fmall plates, though he received these fhadows at the diftance of 56 feet. Nothing was feen but a weak light, equally diffused as in the fhadows of the two fmalleft plates, received very near them. The ftreaks of light in the fhadows of needles of a middling fize, our author afcribed to the rays of light which are inflected at different diftances from the bodies; and he imagined that their croffing one another was fufficient to account for the variations observable in thèm at different diftances. The extraordinary fize of the fhadows of thefe fmall fubftances M. Maraldi thought to be occafioned by the fhadow from the enlightened part of the fky, added to that which was made by the light of the fun, and alfo to a vortex occafioned by the circulation of the inflected light behind the object.

146. M. MARALDI having made the preceding experiments upon fingle long fubftances, had the curiofity to place two of them so as to cross one another in a beam of the fun's light. The fhadows of two hairs placed in this manner, and received at fome distance from them, appeared to be painted reciprocally one upon another, fo that the obfcure part of one of them was vifible upon the ob fcure part of the other. The ftreaks of light allo crofled one another, and the coloured streaks did the fame. Having placed a needle and a hair croífing one another, their fhadows, at the fame dif tance, exhibited the fame appearances as the fhadows of the two hairs, though the fhadow of the needle was the ftronger. He alfo placed in the rays of the fun a bristle and a plate of iron a line

thick, fo that they crossed one another obliquely; and when their fhadows were received at the fame diftance, the light and dark streaks of the fhadow of the briftle were visible so far as the shadow of the plate on the fide of the acute angle, but not on the fide of the obtuse angle, whether the briftle or the plate were placed next to the rays. The plate made a fhadow fufficiently dark, divided into fix black ftreaks; and these were again divided by as many light ones equal to them; and yet all the ftreaks belonging to the fhadow of the briftle were vifible upon it, as in fig. 8.

147. M. MARALDI exposed several small globes in the light of the fun in his dark chamber, and compared their fhadows with thofe of the long fubftances, as he had done in the day-light, and the appearances were ftill fimilar. There was much more light in the shadows of the globes than in thofe of the cylinders, not only when they were both of an equal diameter, but when that of the globe was larger than that of the cylinder, and the fhadows of both the bodies were received at the fame diftance. He could perceive no difference of light in the fhadows of the plates which were a little more than one line broad, though they were received at the distance of 72 feet; but he could eafily fee a difference of fhades in thofe of the globes, taken at the fame diftance, though they were 2 lines in diameter. To explain the colours at the edges of these fhadows, he contrived to throw fome of the fhadows upon others; and the following obfervations, though they did not enable him to accomplish what he intended, are worth reciting: Having thrown feveral of the fimilar colours upon one another, and thereby produced a tinge more lively than before, he threw the gleam of light, which always intervened between the colours and the darker part of the fhadow, upon different parts of other thadows; and obferved, that, when it fell upon the exterior penumbra made by another needle, it produced a beautiful fky-blue colour, almoft like that which was produced by two blue colours thrown to gether. When the fame gleam of light fell upon the deeper fhadow in the middle, it produced a red colour. He placed two plates of iron, each 3 or 4 lines broad, very near one another, but with a very fmall interval between them; and having placed them in the rays of the fun, and received their fhadows at 15 or 20 feet from them, he faw no light between them but a continued fhadow, in the middle of which were fome ftreaks of a lively purple, parallel to one another, and feparated by other black ftreaks; but between them there were other ftreaks, both of a very faint green, and also of a pale yellow.

148. Among thofe who followed Sir Ifaac Newton in his obfervations on the inflection of light, we find the ingenious M. MAIRAN: but he only endeavoured to explain the old facts, by the hypothefis of an atmosphere furrounding all bodies; and confequently making two reflections and refractions of the light that impinges upon them, one at the furface of the atmosphere, and the other at that of the body itself. This atmosphere he fuppofed to be of a variable density and refractive power, like the air.

149. M. DU TOUR thought this variable atmof VOL. XVI. PART I.

phere fuperfluous, and imagined that he could ac count for all the phenomena by an atmosphere of an uniform denfity, and of a lefs refractive power than the air furrounding all bodies. But we are lefs obliged to this gentleman for his ingenious bypothefis, than for the beautiful variety with which he has exhibited the experiments, which render it much easier for any perfon to investigate the true caufes of them. Before he paid attention to this fubject, only 3 fringes had been obferved in the colours produced by the inflection of light; but he observed a greater number, and adopted from Grimaldi the following ingenious method of making them all appear very diftinct. He took a circular board ABED, (Pl. 250, fig. 9.) 13 inches in diameter, the furface of which was black, except at the edge, where there was a ring of white paper about 3 lines broad, in order to trace the circumference of a circle, divided into 360°, beginning at the point A, and reckoning 180° on each hand to the point E; B and D being each of them placed at 90°. A flip of parchment 3 inches broad, in the form of a hoop, was faftened round the board, and pierced at the point E with a square hole, each fide being or 5 lines, to admit a ray of the fun's light. In the centre of the board C, and perpendicular to it, he fixed a pin about of a } line in diameter. This hoop was fo difpofed, that a ray of light entering the dark chamber, through a vertical cleft of 24 lines in length, and about as wide as the diameter of the pin, went through the hole at E, and paffing parallel to the plane of the board, projected the image of the fun and the fhadow of the pin at A. In thefe cir cumftances he obferved,

150. 1. That quite round the concave furface of this hoop, there were a multitude of coloured ftreaks; but that the space mAn, of about 180°, the middle of which was occupied by the image of the fun, was covered with a faint light only. 2. The order of the colours in these streaks was generally fuch that the moft refrangible rays were the nearest to the incident ray ECA; so that, beginning from the point A, the violet was the first and the red the laft colour in each of the ftreaks. In fome of them, however, the colours were difpofed in a contrary order. 3. The image of the fun, projected on each fide of the point A, was divided by the fhadow of the pin, which was bordered by two luminous ftreaks. 4. The coloured ftreaks were narrower in fome parts of the hoop than others, and generally decreased in breadth in receding from the point A. 5. Among these coloured freaks, there were fometimes others which were white, a line or a line and an half in breadth, which were always bordered on both fides by a ftreak of orange colour,at least when the light of the fun was intenfe, and the chamber fufficiently dark.

151. From this experiment he thought it was evident, that the rays which paffed beyond the pin were not the only ones that were decompofed, as thofe which were reflected back from the pin were decompofed alfo; from which he concluded, that they must have undergone fome refraction. He alfo thought that thofe which went beyond the pin fuffered a reflection, fo that they were all af

fected in a fimilar manner. To account for thefe facts, he defcribes the progrefs of a ray of light Xx

through

through an uniform atmosphere, which he fuppofes to furround the pin; and fhows, that the differently refrangible rays will be feparated at their emergence from it. He fhows that the ray a b, g. 10, after being refracted at b, reflected at rand , and again refracted at s and t, will be divided into its proper colours; the leaft refrangible or the red rays iffuing at x, and the moft refrangible or violet at ; which agrees with his obfervations. Thofe ftreaks in which the colours appear in a contrary order he afcribes to inequalities in the furface of the pin.

152. To repeat these experiments, he fays, that the sky must be very clear and free from vapours, in order to exhibit the colours with the greatest diftin&tnefs; fince even the vapours that are imperceptible will diminish the luftre of the colours on every part of the hoop, and even efface fome of them, especially those which are on that part in which the beam of light enters, as at E, Pl. CCL. fg. 9, where the colours are always fainter than in any other place, and indeed can never be diftinguished, except when the hole E is confined by black fubftances, fo as to intercept a part of the light that might reach the pin; and unless alfo thofe rays which go beyond the pin to form the image of the fun at A be ftopped, fo that no rays are vifible except those that are reflected towards the hole, and which make the faint ftreaks. The coloured ftreaks next the fhadow of the pin, he fhows, are formed by those rays which, entering the atmosphere, do not fall upon the pin; and, without any reflection, are only refracted at their entering and leaving the atmosphere, as at b and ru, Plate CCL, fig. 11. In this cafe, the red or leaft refrangible rays will iffue at r, and the violet at u. To diftinguish the rays which fell upon the hoop in any particular direction, from those that came in any other, he made an opening in the hoop, as at P, fig. 9, by which he could, with advantage, and at any diftance from the centre, obferve those rays unmixed with any other. To account for the coloured ftreaks being larger next the fhadow of the pin, and growing narrower to the place where the light was admitted, he shows, by fig. 12, that the rays a b are farther separated by both the refractions than the rays cd. His other obfervations feem to prove that the refracting atmospheres furrounding all kinds of bodies are of the fame fize; for when he placed a great variety of fubftances, and of different fizes alfo, he always found the coloured ftreaks of the fame dimenfions. As the rays which formed thefe coloured ftreaks are but little diverted out of their way, our author infers that this atmosphere is of small extent, and that its refractive power is not much lefs than that of air. Expofing two pieces of paper in the beam of light, fo that part of it paffed between two planes formed by them, M. Du Tour obferved, that the edges of this light, received upon paper, were bordered with two orange-coloured ftreaks, which Newton had not taken notice of in any of his experiments. To account for them, he fuppofes, that, in fig. 13, the more refrangible of the rays which enter at bare fo refracted, that they do not reach the surface of the body itself at R: fo that the red and orange-coloured light may be reflected from thence in the direction M,

where the orange-coloured ftreaks will be formed; and, for the fame reason, another streak of orange will be formed at m, by the rays which enter the atmosphere on the other fide of the chink.

153. But all fuch attempts of philofophers, to explain these phenomena by atmospheres, give no explanation whatever of the phyfical cause of the phenomena. A phenomenon is fome individual fact or event in nature. We are faid to explain it when we point out the general fa& in which it is comprehended, and fhow the manner in which it is fo comprehended, or the particular modification of the general fact. In the prefent inftance there is no general fat referred to. The atmofphere is a mere gratuitous fuppofition; and all that is done is, to fhow a refemblance between the phenomena of inflection of light to what would be the phenomèna were bodies furrounded with fuch atmofpheres; and, even in this point of view, the dif cuffions of Mairan and Du Tour are extremely deficient. They have been fatisfied with very vague refemblances to a fact obferved in one fingle inftance, viz. the refraction of light through the atmosphere of this globe. The attempt is to explain how light is turned out of its direction by paffing near the furface of bodies. This indicates the action of forces in a direction transverse to that of the light. Newton took the right road of inveftigation, by taking the phenomenon in its original fimplicity, and attending merely to this, that the rays are deflected from their former courfe; and the fole aim of his inveftigation was to discover the laws, i. e. the more general facts in this deflection. He deduced from the phenomena, that fome rays are more deflected than others, and endeavoured to determine in what rays the deflections are moft remarkable: and no experiment of M. Du Tour has shown that he was miftaken in his modified affertion, that those rays are most inflected which pass neareft,to the body. We fay modified affertion; for Newton points out with great fagacity many inftances of alternate fits of inflection and deflection; and takes it for granted, that the law of continuity is obferved in thefe phenomena, and that the change of inflection into deflection is gradual. But these analogical difcuffions are eminently deficient in another refpect: They are held out as mechanical explanations of the changes of motion obferved in rays of light. When it fhall be fhown, that these are precifely fuch as are observed in refracting atmof pheres, nothing is done towards deciding the original queftion; for the action of refracting atmof pheres prefents it in all its difficulties, and we muft ftill afk how do these atmospheres produce this effect? No advance whatever is gained in fcience by thrufting in this hypothetical atmosphere; and Newton did wifely in attaching himself to the fimple fact: and he thus gives us another step in fcience, by fhowing us a fact unknown before, viz. that the action of bodies on light is not confined to transparent bodies. He added another general fact to our former stock, that light as well as other matter is a&ed on at a distance; and thus he made a very important deduction, that REFLECTION, REFRACTION, and INFLECTION, are probably brought about by the fame forces.

154. We would extend this obfervation to all

attempte

paffed before his pupil, the fteeple feemed to change its place, and fome hills beyond the fteeple feemed to have the fame motion, juft as if a lens had been drawn betwixt his eye and them. Examining this appearance more attentively, he found that there was a pofition of the wire, but very difficult to keep, in which the fteeple feemed not to have any motion, when the wire was pafed before his eye; and in this cafe, the steeple appeared lefs diftinctly, and feemed to be magnified. Thefe effects being fimilar to thofe of a lens, he attended to them more particularly; and placed his eye in fuch a manner with refpect to the steeple, that the rays of light by which he faw it muft come very clofe to the edge of a window where he had placed himfelf to make his obfervations. Then paffing the wire once more before his eye, he obferved, that, when it was in the vifual axis, the steeple appeared nearer to the window, on whichever fide the wire was made to approach. He repeated this experiment, and conftantly with the fame refult, the object being thus always magnified, and nearly doubled. This phenomenon is eafily explained by Pl. 250, fig. 14. in which B reprefents the eye, A the fteeple, and C the diameter of the wire. The black lines exprefs the cone of light by which the natural image of the fteeple A is formed, and which is much narrower than the diameter of the wire C; but the dotted lines include not only that cone of light, ftopped and turned out of its courfe by the wire, but alfo more diftant rays inflected by the wire, and thereby thrown more converging into the pupil; juft as would have been the effect of the interpofition of a lens between the eye and the object. The refult of this experiment was the fame, whatever fubftances he made ufe of in the place of the wire, provided they were of the fame diameter.

SECT. V. DISCOVERIES concerning VISION. 158. MAUROLYCUS was the firft who showed the true theory of vifion, by demonftrating that the cryftalline humour of the eye, is a lens which collects the light iffuing from external objects, and throws them upon the retina, where is the focus of each pencil. He did not however find out, that, by means of this refraction of the rays, an image of every visible object was formed upon the retina, though this feems hardly to have been a ftep beyond the difcovery he had already made. MONTUCLA indeed, conjectures, that he was prevented from mentioning this part of the difcovery by the difficulty of accounting for the upright appearance of objects, as the image of the retina is always inverted. This difcovery was made by Kepler; but he, too, was much perplexed with the inverted pofition of the image. The rectification of these images, he fays, is the bufinefs of the mind; which, when it perceives an impreffion on the lower part of the retina, confiders it as made by rays proceeding from the higher parts of objects; tracing the rays back to the pupil, where they crofs one another. But this hypothefis can scarcely be deemed fatisfactory. KEPLER did not attempt to account for the manner in which the mind perceives the images upon the retina, and blames Vitellio for attempting prema

turely to determine a queftion of this nature, and which indeed, he says, does not belong to optics. He accounts, however, though not in a fatisfactory manner, from the power we have of seeing diftin&ly at different diftances.

159. The difcovery concerning vifion was completed by SCHEINER. For, in cutting away the coats of the back part of the eyes of theep and oxen, and prefenting several objects before them, within the ufual diftance of vifion, he saw their images diftinctly and beautifully painted upon the retina. He did the fame thing with the human eye, and exhibited this curious experiment at Rome in 1625. He takes particular notice of the refemblance between the eye and the camera obfcura, and explains a variety of methods to make the images of objects erect. As to the images of objects being inverted in the eye, he acquiefces in the reafon given for it by Kepler. He knew that the pupil of the eye is enlarged to view remote objects, and that it is contracted while we are viewing thofe that are near; and this he proved by experiment, and illuftrated by figures. He alfo took a good deal of pains to afcertain the denfity and refractive power of all the humours of the eye, by comparing their magnifying power with that of water or glafs in the fame form and circumftances. The refult of his inquiries was, that the aqueous humour doth not differ much from water in this refpect, nor the cryftalline from glass; and that the vitreous humour is a medium between both. He alfo very accurately and minutely traces the progrefs of the rays of light through all the humours of the eye; and after difcuffing every poffible hypothefis concerning the proper feat of vifion, he demonftrates that it is in the retina, and fhows that this was the opinion of Alhazen, Vitellio, Kepler, and all the moft eminent philofophers. He produces many reafons of his own for this hypothefis; answers a great number of objections to it; and, by a variety of arguments, refutes the opinion of former times, that the feat of vifion is in the crystalline.

160. DESCARTES makes a good number of obfervations on the phenomena of vision. He explains fatisfactorily the natural methods of judging of the magnitudes, fituations, and distances, of objects by the direction of the optic axes; comparing it to a blind man's judging of the fize and diftance of an object, by feeling at it with two fticks of a known length, when the hands in which he holds them are at a known distance from each other. He also obferves, that having been accustomed to judge of the fituation of objects by their images falling on a particular part of the eye; if by any diftortion of the eye they fall on a different place, we are apt to miftake their fituation, or imagine one object to be two; as we imagine one flick to be two, when it is placed between two contiguous fingers laid acrofs one another. But he obferves, that all the methods we have of judging of the diftances of objects are very uncertain, and extend but to narrow limits. The direction of the optic axes, he fays, will not ferve us beyond 15 or 20 feet, and the change of form of the cryftalline not more than 3 or 4 feet. For he imagined that the eye conforms itself to the view of near or diftant objects by a change in the cur