DEPTH OF CHEMICAL Focus. Suppose a small direct pencil of white light to be refracted through a single convex lens. Since white light is not homogeneous, but composed of rays of different refrangibilities, the pencil will be decomposed by refraction through the lens into pencils of the various colours of the spectrum, red, orange, yellow, green, blue, indigo, violet ; and each of these pencils will have its particular focus, the red being furthest from and the violet nearest to the lens. Let r, o, y, g, b, v, be these various foci arranged in the order in which they occur. Then, as the red, orange, and yellow rays have little or no chemical action, that action commencing with the green and terminating with certain invisible ra)rs beyond the violet, the space between g and a little beyond v, is called the DEPTH OF CHEMICAL FOCUS of the lens.
In a single lens this depth of chemical focus is so considerable that the lens cannot be said to have any true chemical focus, and a sitive plate may be placed anywhere within a certain space lying between b and v, and equally good (or equally bad) pictures may be obtained.
When a lens is corrected, as it is Called, for chromatic aberra tion, by combining TWO lenses of different kinds of glass, TWO of the foci, y and v, for instance, are united, and the other foci are brought nearer to this combined focus y v. The depth of chemical
focus is, therefore, greatly diminished, and the point where the maximum of chemical action takes place is also that of the maxi mum of luminosity of the image. When THREE lenses of THREE different kinds of glass are used, THREE of the coloured foci may be combined; and with n lenses n foci.
When the greatest possible amount of sharpness of a photograph is desired, depth of chemical focus must be considered a defect, but when a tolerable average of definition has to be struck between the images of objects at very different distances, a little depth of chemical focus in the lens may be tolerated, because a picture in moderately good focus all over may be considered better than one which is very good in one part, and very bad in another. In general, however, depth of chemical focus must be considered a defect. A single lens is comparatively useless for photographic purposes, because of its great depth of chemical focus.