With regard to the lens required, the practice adhered to by most photographers is to use a lens having a focal length equal to the diagonal of the plate used. Thus for a 1/4-plate camera a 5-inch lens should be used, and for a 1/2-plate an 8-inch lens, and so on. For a 5 × 4 inch camera a 6-inch lens will be required. The following is a simple rule for finding the conjugate foci of a lens, and is useful in obtaining the distance from the lens to the photographic plate and the picture to be copied. Let us suppose that we wish to make a 11/2 times enlarged line negative from a 41/4 × 31/4 inch print. Add 1 to the number of times it is required to enlarge and multiply the result by the focal length of the lens in inches. In the present case this will be 11/2 + 1 = 21/2; and if a 6-inch lens is used, 21/2 × 6 = 15 inches will be the distance of the lens from the plate. Divide this number by the number of times it is desired to enlarge, and the distance of the lens from the picture to be copied is obtained; in this instance 15 ÷ 11/2 = 10 inches. The same rule can be followed when it is required to reduce any given number of times, only in this case the greater number will represent the distance between the lens and the picture to be copied, and the lesser number the distance between the lens and the plate.
In reducing, a 1/4-plate lens will be found to fully cover a 5 × 4 inch plate, providing the reduction is not greater than three to one.
APPENDIX C
LENSES
In this small volume it is not desirable, neither is it intended, to give an exhaustive treatment on the subject of lenses and their action, but as optics plays an important part in the transmission of photographs, both by wireless and over ordinary conductors, the following notes relating to a few necessary principles have been included as likely to prove of interest.
Light always travels in straight lines when in a medium of uniform density, such as water, air, glass, etc., but on passing from one medium to another, such as from air to water, or air to glass, the direction of the light rays is changed, or, to use the correct term, refracted. This refraction of the rays of light only takes place when the incident rays are passed obliquely; if the incident rays are perpendicular to the surface separating the two media they are not refracted, but continue their course in a straight line.
All liquid and solid bodies that are sufficiently transparent to allow light rays to pass through them possess the power of bending or refracting the rays, the degree of refraction, as already explained, depending upon the nature of the body.
The law relating to refraction will perhaps be better understood by means of the following diagram. In Fig. 64 let the line AB represent the surface of a vessel of water. The line CD, which is perpendicular to the surface of the