CORRECTED LENSES.
All glass, when ground into lenses, has the quality of dispersing the different colored rays so that they will not focus at the same point. The rays which ad on the sensitive surface are those from the violet end of the spedrum and are called the adinic or chemical rays. The visual rays are from the red end of the spedrum, hence if these rays are separated by the lens the image we see on the ground glass is not the one which will make the picture. Fortunately flint and crown glass have different dispersive powers and, therefore, one can be made to correct the adion of the other. Hence if we assume that with a non-achromatic lens, such as shown in the cut, the visual focus would be at C, the chemical focus would be at A. Flint glass bends the chemical rays more than does crown glass, and so by combining a negative lens of flint glass with a positive lens of crown glass, the chemical and visual rays are brought into coincidence, z. e., are brought to focus in the same plane as at B. The lens is then called " achromatic." In a non-corrected lens even the chemical rays do not all focus at one point, but in small lenses the dispersion may be so slight as to be impossible of deteCtion in the pidure by the naked eye.
The difficulties of lens making increase, one might say, in geometrical ratio to the size of the plate to be covered, and many forms of lenses other than those shown have been devised to overcome these difficulties, some of which employ as many as six or eight pieces of glass, but these lenses have really no pradical advantage over the simpler forms in such sizes as are commonly used in hand cameras and we shall, therefore, not consider them here.
In very small cameras the best lens is the simplest ; for instance, the lens used in the original Pocket Kodak is a single meniscus non-achromatic, and probably no lens ever had higher encomiums showered upon it. The reason being that it was carefully made, rigidly inspected and tested, and not over-taxed. As a matter of fad, no double achromatic lens could be made that would surpass it in brilliancy or covering power. For larger sizes lenses must be correded but there is no advantage in a double lens unless 33 x 4 or 4 x 5, or larger, pictures are desired. Single lenses tend to slightly curve the marginal lines of the pi&ure, but this is unnoticeable in small pidures and is unobjectionable in view work of any size unless the pi&ures contain architedural subjeCts. Double lenses can be worked with a larger stop opening than single lenses and are, therefore, quicker, especially in the larger sizes. They are almost essential for 5 x 7 and 4 x 5 snap shot work and are preferable for 3 4;.f, but below that the advantage disappears.
Everything depends on the quality of the work put into a lens. Differences of curve or polish not to be detedecl except by an expert often constitute the difference between a good and poor lens and only those which have been through the most rigid inspection and trial should be put in a camera by the manufadurer.