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 lens, or reduce its aperture to a quarter of an inch, as shewn at _a_, we shall have a correct picture of the sitter. Let us now take other _four_ pictures of the same person, by removing 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 2½ 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?[46] [46] 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!