346 HENRY LAURENS AND HENRY D. HOOKER, JR. 



different wave-lengths for other organisms. In our work the 

 energy units are equated as described in the first paper of the 

 series (Laurens and Hooker, '17). The light units' are the 

 figures calculated from rate of movement, or time taken to tra- 

 verse a certain distance, or the presentation or action time, etc. 

 Under these conditions, the sensibility of the organisms to the 

 various lights of different wave-lengths is proportional to the 

 reciprocal of the 'light units.' 



In our first paper (Laurens and Hooker, '17) are set forth 

 additional reasons why it is essential that work of this sort be 

 done with an 'equal-energy spectrum' (also Sheppard, pp 

 102-3). 



It will not be out of place to mention in passing the work 

 which has been done more or less recently, principally by phy- 

 sicists, or from the physicists' point of view, on the visibihty of 

 radiation, or luminosity at constant spectral energy. In this 

 work the distribution of the energy in the spectrum is always 

 taken into consideration. We wish to call attention to the con- 

 tributions to this subject by Nutting ('08), Thiirmel ('10), 

 Ives ('12), Bender ('14), Nutting ('15), Coblentz and Emerson 

 ('17), Luckiesh ('17), Reeves ('18), and Hyde, Forsythe and 

 Cady ('18). It is with these and similar results concerning the 

 relative stimulating value of lights of different wave-lengths for 

 the human eye, in which the luminosity process is considered 

 independently of the color process, that comparisons of the 

 relative stimulating value of light of different wave-lengths for 

 photosensitive organisms, or for photosensitive protoplasm in 

 general, must be made. 



It seems best to avoid the use of the word 'color' in the pres- 

 ent connection, since we are dealing primarily with the objective 

 properties of radiation. Color is a psychological fact, depen- 

 dent upon the integration of physical and physiological bases. 

 In our work on a variety of organisms we are using a physical 

 basis to gain information concerning the physiological basis, 

 and from a strictly objective point of view, for comparison let 

 us say, with the human eye. To say that the physiological 

 effect of radiation consisting of a limited number of wave- 



