192 Orthochromatic Photography. [Sess. 
red rays beyond the red end manifest themselves by the 4 
production of heat, and the invisible rays beyond the violet — 
are evident by their powerful chemical influence. If our — 
ray. of sunlight be admitted through a narrow slit parallel — 
with the length of the prism and be examined with a small — 
telescope, narrow dark bands are seen to cross the spectrum } 
at stated intervals, showing gaps as it were in the continuity — 
of the light. These are the Frauenhofer lines. Their position — 
Fig. 1.—TZhe Solar Spectrum. 
in the spectrum is fixed, and they can be used as reference 
marks. In the absence of actual colour we shall use these — 
lines as our landmarks. 
It is with the chemical power of the various rays of the 
spectrum that we have to do this evening. Chemists soon 
discovered that the salts of silver, under foveal conditional t 
were strongly affected by light. The iodide, bromide, and — 
chloride of silver are the ali usually employed in the manu- ~ 
facture of photographic plates. Freshly precipitated silver 
chloride is white, but if it be exposed for some time to 
ordinary diffused light, it soon becomes dark in colour, and— 
finally turns quite black—the chloride being decomposed, — 
and the silver separating as a black powder. j 
It is worthy of careful note here that the intensity of the 
action of light on the haloid salts of silver depends greatly 
on circumstances. Some substance must be present to 
absorb the liberated chlorine, bromine, or iodine, such as 
nitrate of silver, as in a wet plate, or some organic substance, 
as in the dry plate of to-day. If these substances are not 
