Early Work In Photography
A Text-book For Beginners

Facts about Lenses. 55 This form of lens has two main defects—its pictures suffer from spherical and chromatic aberration, the nature of which we will attempt to explain, first taking chromatic ADEL all the rays were of one color, and therefore equally re- frangible, (i.e. equally capable of being bent) this simple lens would answer many purposes, but we know when vihod beam of light is passed through a prism the rays are divided according to their refrangibility, and instead of getting a spot of light where the rays from the prism fall, we get along vari-colored strip of light—a spectrum. . The same thing occurs with the lens, for all light is composite in its character, and its violet rays, which are more refrangible than the yellow will be focussed, and will give a sharp image at a point nearer the lens. As the violet light rays which are dark to the eye are those which are most active photograph cally,while the yellow light rays (brightest to the eye) are least active we find that we have here two foci (explained later) one called the visual, and the other the chemical or actinic focus. If we place our ground glass screen (or later our sensitive plate) at the point where the visual image is strongest and sharpest, we shall find, on developing the photographic image, that it is feeble and blurred. If we move the screen (and plate) forward until the visual image is somewhat blurred, we shall find that the photographic image on development, is better defined. This class of truly single lens is sometimes used bv photographers of the impressionist school, and they find i?necessary, after focussing their image on the screen, to rack in the camera back to an extent ascertained by experi- mSpherical aberration is rather more difficult to explain. In the last diagram we shewed all the rays from (say) the tip Fig. 15. of candle-frame, coming to one point on the ground-glass, whether they passed through the centre or the margin of the lens But this diagram was falsely drawn, merely to illustrate a point in passing; and what actually happens would be better represented by the above diagram, which