V represents the focus of the violet ray, Y of the yellow, and E of the red.
Hence, as the chemical action corresponds more to the violet, the most marked actinic effect would be produced at V. The luminous portion of the spectrum however is the yellow, consequently the visual focus is at Y.
Photographers have long recognized this point; and therefore, with ordinary Lenses, not corrected for colour, rules are laid down as to the exact distance which the sensitive plate should be shifted away from the visual focus in order to obtain the greatest amount of distinctness of outline in the image impressed by chemical action.
These rules do not apply to the Achromatic Lenses recently described. The coloured rays being in that case bent together again and reunited, the two foci also nearly correspond. By a little further correction to a point higher in the Spectrum, they are made to do so perfectly.
SECTION IV.
On the Photographic Action of Coloured Light.
It has already been mentioned in the First Section of this Chapter that certain of the elementary colours of white light, viz. the violet and indigo, are peculiarly active in decomposing the Photographic Salts of Silver; but there are some points of importance relating to the same subject which require a further notice.
The term "actinism" (Gr. ἁκτἱς, a ray or flash) has been proposed as convenient to designate the property possessed by light of producing chemical change; the rays to which the effect is especially due being known as actinic rays.
If the pure Solar Spectrum formed by prismatic analysis in the manner represented at [page 47] be allowed to impinge upon a prepared sensitive surface of Iodide of Silver, the latent image being subsequently developed by a reducing agent, the effect produced will be something similar to that represented in the following diagram:—
| Fig. 1. | Fig. 2. |