364 HENRY LAURENS AND HENRY D. HOOKER, JR. 



tensity would be found to be equally efficient as stimuli, because 

 the absolute maximum effect of the wave-lengths of greatest 

 stimulating effect has been reached at a lower intensity of the 

 spectrum, while that of the other wave-lengths was below their 

 possible maximal effect, or because of the influence of some lim- 

 iting factor. 



It is probably also true that if the absolute intensity of an 

 equal-energy spectrum be decreased, certain wave-lengths, which 

 at a greater intensity have a relatively weak stimulating efficiency, 

 will, as the intensity is decreased, have finally none at all, and 

 this will spread and thus involve more and more of the spectrum. 

 But the wave-lengths of maximum effect will remain the same 

 for all intensities. 



This is of course pure assumption. We know of no work on 

 the influence of the relative intensity of the spectrum on the 

 location of the wave-lengths of maximum stimulating efficiency 

 for other photosensitive protoplasm than the human eye. But 

 a low-intensity spectrum has for the human eye a region of max- 

 imum stimulating efficiency (achromatic scotopic luminosity 

 curve) nearer the blue end of the spectrum than a high-intensity 

 spectrum (photopic luminosity curve), and its luminosity curve 

 is similar to that of the totally color-blind under all intensities 

 (Parsons, pp. 189, 209), as well as to that of rod vision (peripheral 

 vision) under high illumination (according to Bender, '14, figs. 

 2 and 4), the curve for the peripheral retina coinciding with that 

 of the foveal visibility curve of totally color-blind persons, with 

 maxima at XSlSni/x. (The question of the quantitative and quali- 

 tative differences between peripheral vision and scotopia on the 

 one hand and central vision on the other, either need revision or 

 reinvestigation. Parsons, pp. 71-72.) 



Now it is a question, as above indicated, whether anything 

 comparable can be found in the sensibility to different wave- 

 lengths in lower organisms. Can we, by varying the absolute 

 intensity, keeping the spectrum equal in energy throughout, shift 

 the region of maximum stimulating efficiency? Probably not. 

 But it is a question well worth considering and investigating. 

 If the luminosity curve for the peripheral retina (normal rod 



