slightly yellow color, transmitting freely almost all the colored rays. When melted, and suddenly cooled, it acquired the power of absorbing all the colors of the spectrum at thicknesses at which it formerly transmitted them all. The blackness produced upon pure phosphorus was first observed by Thenard. Mr. Faraday observed, that glass tinged purple with manganese had its absorptive power altered by the mere transmission through it of the solar rays.

By the method above described of absorbing particular colors in the spectrum, I was led to propose a new method of analyzing white light. The experiments with the blue glass incontestably prove that the orange and green colors in solar light are compound colors, which, though they cannot be decomposed by the prism, may be decomposed by absorption, by which we may exhibit alone the red part of the orange and the blue part of the green, or the yellow part of the orange and the yellow part of the green; and, by submitting the other colors of the spectrum to the scrutiny of absorbent media, I was led to the conclusions respecting the spectrum which are explained in Chapter VII.

We have already seen that in the solar spectrum, as described by Fraunhofer, there are dark lines, as if rays of particular refrangibilities had been absorbed in their course from the sun to the earth. The absorption is not likely to have taken place in our atmosphere, otherwise the same lines would have been wanting in the spectra from the fixed stars, and the rays of solar light reflected from the moon and planets would probably have been modified by their atmospheres. But as this is not the case, it is probable that the rays which are wanting in the spectrum have been absorbed by the sun's atmosphere, as Mr. Herschel has supposed.

(88.) Connected with the preceding phenomena is the subject of colored flames, which, when examined by a prism, exhibit spectra deficient in particular rays, and resembling the solar spectrum examined by colored glasses. Pure hydrogen gas burns with a blue flame, in which many of the rays of light are wanting. The flame of an oil lamp contains most of the rays which are wanting in sun-light. Alcohol mixed with water, when heated and burned, affords a flame with no other rays but yellow. Almost all salts communicate to flames a peculiar color, as may be seen by introducing the powder of these salts into the exterior flame of a candle, or into the wick of a spirit lamp. The following results, obtained by different authors, have been given by Mr. Her

schel:

According to Mr. Herschel the muriates succeed best on account of their volatility.

CHAP. XVII.

ON THE DOUBLE REFRACTION OF LIGHT.

(89.) In the preceding chapters of this work it has always been supposed, when treating of the refraction of light, either through surfaces, lenses, or prisms, that the transparent or refracting body had the same structure, the same temperature, and the same density in every part of it, and in every direction in which the ray could enter it. Transparent bodies of this kind are gases, fluids, solid bodies, such as different kinds of glass, formed by fusion, and slowly and equally cooled, and a numerous class of crystallized bodies, the form of whose primitive crystal is the cube, the regular octohedron, and the rhomboidal dodecahedron. When any of these bodies have the same temperature and density, and are not subject to any pressure, a single pencil of light incident upon any single surface of them, perfectly plane, will be refracted into a single pencil according to the law of the sines explained in Chapter III.

In almost all other bodies, including salts and crystallized minerals not having the primitive forms above mentioned; animal bodies, such as hair, horn, shells, bones, lenses of animals and elastic integuments; vegetable bodies, such as certain leaves, stalks, and seeds; and artificial bodies, such as resins, gums, jellies, glasses quickly and unequally cooled, and solid bodies having unequal density either from unequal temperature or unequal pressure;-in all such bodies a single pencil of light incident upon their surfaces will be refracted into two different pencils, more or less inclined to one another, according to the nature and state of the body, and according to the direction in which the pencil is incident. The separation of the two pencils is sometimes very great, and in most cases easily observed and measured; but in other cases it is

not visible, and its existence is inferred only from certain effects which could not arise except from two refracted pencils. The refraction of the two pencils is called double refraction, and the bodies which produce it are called doubly refracting bodies or crystals.

As the phenomena of double refraction were first discovered in a transparent mineral substance called Iceland spar, calcareous spar, or carbonate of lime, and as this substance is admirably fitted for exhibiting them, we shall begin by explaining the law of double refraction as it exists in this mineral. Iceland spar is composed of 56 parts of lime, and 44 of carbonic acid. It is found in almost all countries, in crystals of various shapes, and often in huge masses; but, whether found in crystals or in masses, we can always cleave it or split it into shapes like that represented in fig. 70., which is called a

E

Fig. 70.

rhomb of Iceland spar, a solid bounded by six equal and similar rhomboidal surfaces, whose sides are parallel, and whose angles BAC, ACD are 101° 55' and 78° 5'. The inclination of any face ABCD to any of the adjacent faces that meet at A is 105o 5', and to any of the adjacent faces that meet at X 74° 55'. The line A X, called the axis of the rhomb or of the crystal, is equally inclined to each of the six faces at an angle of 45° 23′. The angle between any of the three edges, BA, CA, EA, that meet at A, or of the three that meet at X, and the axis A X is 66° 44′ 46′′, and the angle between any of the six edges and the faces is 113° 15' 14" and 66° 44' 46".

B

(90.) Iceland spar is very transparent, and generally colorless. Its natural faces, when it is split, are commonly even and perfectly polished; but when they are not so, we may, by a new cleavage, replace the imperfect face by a better one, or we may grind and polish any imperfect face.

Having procured a rhomb of Iceland spar like that in the figure, with smooth and well polished faces, and so large that one of the edges A B is at least an inch long, place one of its faces upon a sheet of paper, having a black line M N drawn upon it, as shown in fig. 71. If we then look through the upper surface of the rhomb with the eye about R, we shall probably see the line M N double; but if it is not, it will become double by turning the crystal a little round. Two lines, MN, m n, will then be distinctly visible; and upon turning the crystal round, preserving the same side always upon the paper, the two lines will coincide with one another, and appear to form one at two opposite points during

a whole revolution of the crystal; and at two other opposite points, nearly at right angles to the former, the lines will be at their greatest distance. If we place a black spot at O, or a luminous aperture, such as a pin-hole in a wafer, with light passing through the hole, the spot or aperture will appear Fig. 71.

M

B

R

double, as at O and E; and by turning the crystal round as before, the two images will be seen separate in all positions; the one, E, revolving, as it were, round the other, O.

Let a ray or pencil of light, R r, fall upon the surface of the rhomb at r, it will be refracted by the action of the surface into two pencils, r O, r E, each of which, being again refracted at the second surface at the points O, E, will move in the directions O o, E e, parallel to one another and to the incident ray Rr. The ray Rr has therefore been doubly refracted by the rhomb.

If we now examine and measure the angle of refraction of the ray r O corresponding to different angles of incidence, we shall find that, at 0° of incidence, or a perpendicular incidence, it suffers no refraction, but moves straight through the crystal in one unbroken line; that at all other angles of incidence the sine of the angle of refraction is to that of incidence as 1 to 1.654; and that the refracted ray is always in the same plane as that of the incident ray. Hence it is obvious that the ray r O is refracted according to the ordinary law of refraction, which we have already explained. If we now examine in the same way the ray r E, we shall find that, at a perpendicular incidence, or one of 0°, the angle of refraction, in place of being 0°, is actually 6° 12'; that at other incidences the angle of refraction is not such as to follow the constant ratio of the sines; and, what is still more extraordinary, that the refracted ray r E is bent to one side, and lies entirely out of the plane of incidence. Hence it follows that

the pencil r E is refracted according to some new and extraordinary law of refraction. The ray r O is therefore called the ordinary ray, and r E the extraordinary ray.

If we cause the ray Rr to be incident in various different directions, either on the natural faces of the rhomb or on faces cut and polished artificially, we shall find that in Iceland spar there is one direction, namely, A X, along which, if the refracted pencil passes, it is not refracted into two pencils, or does not suffer double refraction. In other crystals there are two such directions, forming an angle with each other. In the former case the crystal is said to have ONE AXIS of double refraction, and in the latter case TWO AXES of double refraction. These lines are called axes of double refraction, because the phenomena are related to these lines. In some bodies there are certain planes, along which, if the refracted ray passes, it experiences no double refraction.

but

An axis of double refraction, however, is not, like the axis of the earth, a fixed line within the rhomb or crystal. It is only a fixed direction: for if we divide, as we can do, the rhomb A B C, fig. 70., into two or more rhombs, each of these separate rhombs will have their axes of double refraction; when these rhombs are again put together, their axes will be all parallel to A X. Every line, therefore, within the rhomb parallel to A X, is an axis of double refraction; but as these Îines have all one and the same direction in space, the crystal is still said to have only one axis of double refraction.

In making experiments with different crystals, it is found that in some the extraordinary ray is refracted towards the axis A X, while in others it is refracted from the axis A X. In the first case the axis is called a positive axis of double refraction, and in the second case a negative axis of double refraction.

On Crystals with one Axis of Double Refraction.

(91.) In examining the phenomena of double refraction in a great number of crystallized bodies, I found that all those crystals whose primitive or simplest form had only ONE AXIS of figure, or one pre-eminent line round which the figure was symmetrical, had also ONE AXIS of double refraction; and that their axis of figure was also the axis of double refraction. The primitive forms which possess this property are as follows:-The rhomb with an obtuse summit. The rhomb with an acute summit. The regular hexahedral prism. The octohedron with a square base. The right prism with a square base.