296 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



The phenomenon transpires quite differently if the observations are 

 made so that only the rods or only the cones can be affected. In the 

 first place, the intensity of the gray sensation would have to increase 

 with increasing temperature without ever passing to redness. We 

 might expect to observe this state if the radiations are confined to the 

 outer portions of the retina, where tlfe rods predominate. 



If, however, a very small surface be observed so that only the yellow 

 spot, where there are no rods, is affected, the gray stage would be entirely 

 eliminated, and the surface would, when first visible, appear red. 



A few observations I have personally made verify on the whole these 

 deductions. It is very easy to show that a small platinum surface 

 heated electrically gives out a grayish light by indirect vision before it 

 is visible at all by direct vision. The following experiment will illustrate 

 this more clearly: 



I look at a surface of molten niter (temperature 550° C.) through a 

 small hole in the cover of the bath. The conditions are such that very 

 close to the hole the whole surface is simultaneously visible, while at 

 greater distances the visible portion is limited by the size of the 

 opening. 



In accordance with theory the reddish glow of the surface, seen from 

 the distance, assumes on approaching the eye a whitish appearance, 

 until finally there remains hardly a trace of color in the sensation pro- 

 duced. 



The following experiment indicates the far greater sensitiveness of 

 the rods at small intensities. If a small opening in a hollow copper 

 sphere, heated to 600° C, is viewed by direct vision, it appears bright 

 red, corresponding almost to the radiation emitted by an absolutely 

 black body at the same temperature. But by directing the eye slightly 

 to one side the opening appears to emit the colorless, grayish light, and 

 assumes the appearance of the full moon in the heavens on a starry 

 night. 



These experiments have been repeated, and their results confirmed 

 by Prof. E. Pringsheim. 



PHYSICAL BASIS OP CLASSIFICATION OF SOURCES OF LIGHT. 



As soon as the initial stages have been passed, the intensity increases 

 very rapidly with increasing temperature, at the same time taking on 

 more and more of a color such as is produced when we add to red yel- 

 low, then green, then blue, and finally violet. At a sufiBciently high 

 temperature the light produces the sensation of white and the tem- 

 perature is called white heat. You can follow the various stages of 

 luminosity produced by gradually heating this strip of jjlatinum up 

 to its melting point. [Experiment.] Immediately before melting, the 

 strip emits a fullness of white light which makes this large hall almost 

 as bright as day. But nevertheless the temperature corresponding to 

 bright redness is a little less than half of that corresponding to white- 



