| 1900-1901. ] Orthochromatic Photography. 193 
| present the action of light becomes comparatively slight— 
| so that you will observe that these haloid salts of silver 
really need a sensitiser for white light. This is worthy of 
| particular notice, because we shall presently see that another 
| sensitiser applied to these salts has a further effect in making 
them sensible to light rays in a yet greater degree. 
| Now, when light acts on a photographic plate the silver 
| salts contained in the film are modified, but the effect is 
not visible to the eye. Certain chemicals, however, when 
| applied to the plate, cause a separation of silver from the 
portions affected by light, the amount of deposit depending 
upon the intensity of the illumination. Here is a plate 
| which has been exposed in the camera to a picture of a white 
| eross on a black ground. You see it appears to have been 
| unaffected. Here is a duplicate plate which has been ex- 
posed to the same subject and developed and fixed. The 
part affected by light is now clearly seen, and you will 
| observe that the white cross is represented by a black 
| deposit. The white light from the cross has affected the 
| silver salt in the film, and the developer has caused the 
| silver to separate as a black deposit. This is a negative. 
The next slide shows a print from the same in the usual way. 
| Now we must return to the spectrum. Having seen by 
the foregoing simple experiment that ordinary white light 
| affects the photographic film, we may now inquire which 
of the various colours that go to make up white light have 
the most powerful effect. This diagram (fig. 2) is designed 
al 
Sefer np oe 
Fig. 2.—Colour Effects in Photography. 
to give us the information we require. Here we see the 
Spectrum drawn with its various colours in their proper 
