2 Prof. S. P. Langley on Energy and Vision. 



investigations of the present writer. Still the fact that there 

 could be such a misapprehension at the present day led me to 

 look at the matter again, and to observe, with some surprise, 

 that there was nowhere, in any physical work known to me, 

 any exact or even approximately exact statement of the rela- 

 tive ocular effects of a given amount of energy in different 

 parts of the spectrum. I have undertaken, therefore, during 

 the last few months an experimental reinvestigation of this 

 subject, with such a statement especially in view. 



We shall evidently need two correlated sets of experiments; 

 the first set to determine the amount of energy in each ray, 

 the second to show the corresponding visual effect. 



For the first of these, since energy only shows itself through 

 absorptive media which more or less disguise it, we must 

 select that manifestation which disguises it least ; and in this 

 respect beyond comparison the thermal one stands first, as the 

 heat dispersed by a glass prism and shown by a thickly lamp- 

 blacked thermometric apparatus is, throughout the visible 

 spectrum, very nearly proportionate to the energy itself. For 

 these first or thermal experiments, whence the energy is 

 readily deduced, with close approximation, we shall rely prin- 

 cipally upon a very elaborate investigation made here some 

 time since and already published, where the bolometer is used 

 to deduce in terms of lampblack absorption the relative 

 amounts of solar energy in various wave-lengths throughout 

 the visible spectrum and a little beyond ; and which has been 

 supplemented by a new investigation of the same kind in the 

 present connexion. 



Our second set of experiments will consist of a recent 

 parallel series of photometric solar measurements taken at the 

 same wave-lengths as the thermal ones, and which we may 

 say gives this energy in terms of what I may perhaps be 

 allowed to call, provisionally, " retinal " absorption. 



The thickly lampblacked sui'face, then, and the retinal 

 screen provided by nature in the eye, both exercise selective 

 absorption ; but the first, whose absolute absorption is here 

 nearly total, does so in relatively so small a degree that we 

 may, in the visible spectrum, provisionally neglect it, and 

 consider the bolometric effect as here proportional to the 

 energy itself. 



It is evident that these two series once made, and reduced 

 in both cases to the normal spectrum, would give us for any 

 individual human eye the means of stating the visual effect in 

 terms of absolute energy. The visual effect is known to vary 

 in a very minute degree with the absolute amount of this 



