inch in diameter in the hole H, the distinctness of the picture will be restored, and, from the introduction of so much light, the photograph may be completed in a sufficiently short time. The lens should be made of rock crystal, which has a small dispersive power, and the ratio of curvature of its surfaces should be as six to one, the flattest side being turned to the picture. In this way there will be very little colour and spherical aberration, and no error produced by any striæ. or want of homogeneity in the glass.

As the hole H is nearly the same as the greatest opening of the pupil, the picture which is formed by the enclosed lens will be almost identical with the one we see in monocular vision, which is always the most perfect representation of figures in relief.

With this approximately perfect camera, let us now compare the expensive and magnificent instruments with which the photographer practises his art. We shall suppose his camera to have its lens or lenses with an aperture of only three inches, as shewn at LR in Fig. 45. If we cover the whole

Let us

lens, or reduce its aperture to a quarter of an inch, as shewn at a, we shall have a correct picture of the sitter. now take other four pictures of the same person, by re

moving the aperture successively to b, c, d, and e: It is obvious that these pictures will all differ very perceptibly from each other. In the picture obtained through d, we shall see parts on the left side of the head which are not seen in the picture through c, and in the one through c, parts on the right side of the head not seen through d. In short, the pictures obtained through c and d are accurate dissimilar pictures, such as we have in binocular vision, (the distance cd being 2 inches,) and fitted for the stereoscope. In like manner, the pictures through b and e will be different from the preceding, and different from one another. In the one through b, we shall see parts below the eyebrows, below the nose, below the upper lip, and below the chin, which are not visible in the picture through e, nor in those through c and d; while in the picture through e, we shall see parts above the brow, and above the upper lip, &c., which are not seen in the pictures through b, c, and d. In whatever part of the lens, LR, we place the aperture, we obtain a picture different from that through any other part, and therefore it follows, that with a lens whose aperture is three inches, the photographic picture is a combination of about one hundred and thirty dissimilar pictures of the sitter, the similar parts of which are not coincident; or to express it in the language of perspective, the picture is a combination of about one hundred and thirty pictures of the sitter, taken from one hundred and thirty different points of sight! If such is the picture formed by a three-inch lens, what must be the amount of the anamorphism, or distortion of form, which is produced by photographic lenses of diameters from three to twelve inches, actually used in photography ?1

1 See my Treatise on Optics, 2d edit., chap. vii. p. 65.

But it is not merely by the size of the lenses that hideous portraits are produced. In cameras with two achromatic lenses, the rays which form the picture pass through a large thickness of glass, which may not be altogether homogeneous, through eight surfaces which may not be truly spherical, and which certainly scatter light in all directions,and through an optical combination in which straight lines in the object must be conic sections in the picture!

Photography, therefore, cannot even approximate to perfection till the artist works with a camera furnished with a single quarter of an inch lens of rock crystal, having its radii of curvature as six to one, or what experience may find better, with an achromatic lens of the same aperture. And we may state with equal confidence, that the photographer who has the sagacity to perceive the defects of his instruments, the honesty to avow it, and the skill to remedy them by the applications of modern science, will take a place as high in photographic portraiture as a Reynolds or a Lawrence in the sister art.

Such being the nature of single portraits, we may form some notion of the effect produced by combining dissimilar ones in the stereoscope, so as to represent the original in relief. The single pictures themselves, including binocular and multocular representations of the individual, must, when combined, exhibit a very imperfect portrait in relief,—so imperfect, indeed, that the artist is obliged to take his two pictures from points of sight different from the correct points, in order to produce the least disagreeable result. This will appear after we have explained the correct method of taking binocular portraits for the stereoscope.

No person but a painter, or one who has the eye and the

taste of a painter, is qualified to be a photographer either in single or binocular portraiture. The first step in taking a portrait or copying a statue, is to ascertain in what aspect and at what distance from the eye it ought to be taken.

In order to understand this subject, we shall first consider the vision, with one eye, of objects of three dimensions, when of different magnitudes and placed at different distances. When we thus view a building, or a full-length or colossal statue, at a short distance, a picture of all its visible parts is formed on the retina. If we view it at a greater distance, certain parts cease to be seen, and other parts come into view; and this change in the picture will go on, but will become less and less perceptible as we retire from the original. If we now look at the building or statue from a distance through a telescope, so as to present it to us with the same distinctness, and of the same apparent magnitude as we saw it at our first position, the two pictures will be essentially different; all the parts which ceased to be visible as we retired will still be invisible, and all the parts which were not seen at our first position, but became visible by retiring, will be seen in the telescopic picture. Hence the parts seen by the near eye, and not by the distant telescope, will be those towards the middle of the building or statue, whose surfaces converge, as it were, towards the eye; while those seen by the telescope, and not by the eye, will be the external parts of the object, whose surfaces converge less, or approach to parallelism. It will depend on the nature of the building or the statue which of these pictures gives us the most favourable representation of it.

If we now suppose the building or statue to be reduced

in the most perfect manner,—to half its size, for example,— then it is obvious that these two perfectly similar solids will afford a different picture, whether viewed by the eye or by the telescope. In the reduced copy, the inner surfaces visible in the original will disappear, and the outer surfaces become visible; and, as formerly, it will depend on the nature of the building or the statue whether the reduced or the original copy gives the best picture.

If we repeat the preceding experiments with two eyes in place of one, the building or statue will have a different appearance; surfaces and parts, formerly invisible, will become visible, and the body will be better seen because we see more of it; but then the parts thus brought into view being seen, generally speaking, with one eye, will have less brightness than the rest of the picture. But though we see more of the body in binocular vision, it is only parts of vertical surfaces perpendicular to the line joining the eyes that are thus brought into view, the parts of similar horizontal surfaces remaining invisible as with one eye. It would require a pair of eyes placed vertically, that is, with the line joining them in a vertical direction, to enable us to see the horizontal as well as the vertical surfaces; and it would require a pair of eyes inclined at all possible angles, that is, a ring of eyes 2 inches in diameter, to enable us to have a perfectly symmetrical view of the statue.

These observations will enable us to answer the question, whether or not a reduced copy of a statue, of precisely the same form in all its parts, will give us, either by monocular or binocular vision, a better view of it as a work of art. As it is the outer parts or surfaces of a large statue that