380 WAVE-LENGTH AND PHOTOSENSORY PROCESS 



The distribution of the energy transmitted by each screen is given 

 in Fig. 3. For uniformity the total transmission is given the value of 

 100. It will be seen that the point of maximum energy transmission 

 is occasionally shifted toward the red end as compared with the 

 per cent transmission given in Fig. 1. This is, of course, because the 

 energy content of the spectrum increases steadily toward the red end, 

 as shown in Fig. 2. 



3. In Figs. 1 and 3 the transmission of the filters is given only for 

 the visible spectrum. All the filters transmit infra-red rays. It may, 

 however, be said at once that rays beyond X = 680 ju/x are ineffective 

 as stimulating agents for Mya. I have determined this by subjecting 

 animals to very intense illumination from which all the visible rays 

 have been filtered out by means of Wratten Light Filter No. 87. 

 This screen transmits the rays beyond X = 680 m,«, and none below 

 that wave-length. In no case did the animals respond to these infra- 

 red rays. As a source of error, therefore, infra-red rays may be left 

 out of consideration. 



The screens do not transmit ultra-violet light. In addition, the 

 light from the incandescent filament has to pass through several 

 centimeters of glass before reaching the animal. Therefore, the ultra- 

 violet rays are screened out in this way as well, and do not enter as a 

 source of error. 



The filters are quite photostable. But in order to avoid any pos- 

 sible bleaching effect, a shutter is placed between the light source and 

 the filter, so that the filter is exposed to light only during the few 

 seconds required to expose an animal and to measure its reaction time. 

 Moreover, the light is turned off between exposures in order that as 

 little heat be formed as possible. This is a necessary precaution, 

 because a 400 watt lamp produces a tremendous amount of heat, 

 which might otherwise pass even through the shutter and affect the 

 gelatin between the glass plates of the filter. 



From what has been said it is evident that these Wratten Light 

 Filters furnish a simple method of procuring practically monochro- 

 matic light of known intensities, many times greater than can ordi- 

 narily be produced with the aid of a dispersion spectrum. 



4. Further than the use of the screens, the experiments involve no 

 new principle. A filter is placed in the path of the beam of light im- 



