360 S. P. Langley — Energy and Vision. 



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 connection. 



Our second set of experiments will consist of a recent paral- 

 lel s'eries of photometric solar measures 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, pro- 

 visionally, "retinal" absorption. 



The thickly lamp-blacked surface, then, and the retinal screen 

 provided by nature in the eye, both exercise selective absorp- 

 tion, but the first whose absolute absorption is here nearly total 

 does so in relatively so small a degree, that we may, in the visi- 

 ble spectrum, provisionally neglect it, and consider the bolo- 

 metric 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 indi- 

 vidual 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 

 energy, at least if we admit the physiological influence of what 

 has been called "the color of brightness," but for the compara- 

 tively feeble lights employed, this physiological effect seems to 

 be almost negligible, and it is nearly immaterial within the 

 limits of the experiment what unit of energy we take. 



The object of these experiments, then, is to take some one 

 constant amount of energy, to actually or virtually display it 

 successively in different portions of the spectrum, and to ob- 

 serve in what proportion the optical or visual effects of this 

 fixed amount of energy vary, according to the wave-length in 

 which it is conveyed. While the measurements which insure 

 this constancy are best made by thermal methods, and while 

 the prism is on the whole far more convenient for them than 

 the grating, it is nevertheless desirable to reduce the whole 

 measurements to what they would have been, if taken directly 

 in the normal spectrum. The writer's measurements, already 

 published and here cited later, afford the means of doing this 

 with precision. These show that the energy is far from being 

 distributed equally even in the normal spectrum ; and that, 

 accordingly as it varies from one part of the spectrum to an- 

 other, we must, by opening the aperture through which it is 

 admitted where it is weak, and by narrowing it where the ener- 

 gy is strong, or by other like device, maintain it absolutely con- 

 stant, or else (what is far better) let it enter through one fixed 

 aperture, and use the subjoined table to apply a correction for 

 the actual irregularities. Let it be remembered that we are 

 now speaking of absolute energy, not of those physiological 

 effects of it on the organ of vision which we call light, and it is 



