Prof. S. P. Langley on Energy and Vision. 3 



energy, at least if we admit the physiological influence of 

 what has been called "the colour of brightness ;" but for the 

 comparatively 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 

 observe 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, 

 according as it varies from one part of the spectrum to 

 another, we must, by opening the aperture through which it 

 is admitted w^here it is weak, and by narrowing it where the 

 energy is strong, or by other like deface, maintain it abso- 

 lutely constant, or else (what is far better) let it enter through 

 one fixed aperture, and use the subjoined table to apply a cor- 

 rection for the actual irrecrularities. 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 w^e call 

 light ; and it is to the value of this absolute energy for dif- 

 ferent wave-lengths in the normal spectrum which the sub- 

 joined table refers. This table, which gives the energy as 

 derived from thermal experiments, rests on many thousand 

 observations, taken, however, all wdth what is called a high 

 sun, i. e. with a sun more than 30° above the horizon. As 

 the distribution of this energy varies somewhat from day to 

 day, and particularly in the violet and beyond, we have sup- 

 plemented it by a series of direct observations taken with the 

 bolometer on April 6, 1888, using the same glass prism em- 

 ployed in the photometric work described later. As those 

 observations show a fair accordance with the others, it is not 

 necessary to repeat them. 



B2 



