304 On the Measurement of the _ (duly, 
is as distinct from photometry proper as is the thermometric¢ regis- 
tration of the heat rays constituting the other end of the spectrum. 
What we want is a method of measuring the intensity of those rays 
which are situated at the intermediate parts of the spectrum, and 
produce in the eye the sensation of light and colour; and, as pre- 
viously suggested, there is a reasonable presumption that further 
researches may place us in possession of a photometric method based 
upon the chemical action of the 7wminous rays of light. 
The rays which affect an ordinary photographic sensitive sur- 
face, are so constantly spoken of and thought about as the ultra-violet 
invisible rays, that it is apt to be forgotten that some of the highly 
luminous rays of light are capable of exerting chemical action. 
Fifteen years ago the writer was engaged in some investigations on 
the chemical action of light, and he succeeded in producing all the 
ordinary phenomena of photography, even to the production of good 
photographs in the camera, by purely luminous rays of light free 
from any admixture with the violet and invisible rays. When the 
solar spectrum, of sufficient purity to show the principal fixed lines, 
is projected for a few seconds on to a sensitive film of iodide of 
silver, and the latent image then developed, the action is seen to 
extend from about the fixed line G to a considerable distance into 
the ultra-violet invisible rays. When the same experiment was 
repeated with a sensitive surface of bromide of silver instead of 
iodide of silver, the result of the development of the latent image 
showed that in this case the action commenced at about the fixed 
line b, and extended, as in the case of the iodide of silver, far beyond 
the violet. A transparent cell, with parallel glass sides one inch 
across, was filled with a solution of twenty-five parts of sulphate of 
quinine to one hundred parts of dilute sulphuric acid ; this was placed 
across the path of the rays of light, and photographs of the spec- 
trum were again taken on iodide of silver and on bromide of silver, 
the arrangements in all cases being identical with those in the first 
cited experiments, with the exception of the interposition of the 
quinine screen. The action of the sulphate of quinine upon a ray 
of light is peculiar ; to the eye it scarcely appears to have any action 
at all, but it is absolutely opaque to the ultra-violet, so called chemi- 
cal, rays, and thus limits the photographic action on the bromide 
and iodide of silver to the purely luminous rays. On developing 
the latent images, it was now found that the action on iodide of 
silver was confined to a very narrow line of rays, close to the fixed 
line G, and in the case of bromide of silver to the space between b 
and G. Designating the spaces of action by colours instead of fixed 
lines, it was thus proved that, behind a screen of sulphate of quinine, 
iodide of silver was affected only by the luminous rays about the 
centre of the indigo portion of the spectrum, whilst bromide of silver 
was affected by the green, blue, and some of the indigo rays. 
