378 WAVE-LENGTH AND PHOTOSENSORY PROCESS 



have been carefully measured.^ They are presented graphically in 

 Fig. 1. The white spaces are the transmission areas for the different 

 screens in terms of the per cent transmission of the incident light. 



It is apparent that each screen transmits a rather narrow portion 

 of the visible spectrum. In the significant part of the spectrum (X = 

 400 to 600 mm) the screens transmit bands about 40 mm wide; most of 

 the light transmitted is confined to an even narrower band. 



2. With these screens I used a 400 watt, concentrated-filament 

 Mazda lamp, running on an ordinary lighting circuit of 115 volts. 

 This source of light may be considered as a point source, and different 

 relative intensities may be obtained by placing animals at different 

 distances from the light, the intensities being computed on the 

 inverse square law. 



For ordinary sources of light the energy content of the spectrum 

 varies with the wave-length. It is therefore important to know the 

 exact values of the relative energy distribution of the light in order 

 that the energy transmitted by the filters be known. The results of 

 the determination of the energy distribution for the 400 watt lamp 

 which I used in these experiments are shown in Fig. 2, which gives the 

 relative energy content of the different wave-lengths of the visible 

 spectrum. 



From Fig. 2 we know the energy incident on the filters. Fig. 1 in 

 turn tells us the portion of this incident energy which is transmitted 

 by each of the filters. It is thus a simple computation to find out the 

 energy content of the light transmitted by each filter in conjunction 

 with the lamp. The transmission values in Fig. 1 and the energy 

 values in Fig. 2 are both determined for bands 10 mm wide. The total 

 energy transmitted by any filter is therefore the sum of the energy 

 transmitted by its constituent 10 mm bands. These totals are given in 

 Table I. 



The Mazda lamp plus a screen may thus be considered as a source 

 of energy which radiates a narrowly defined portion of the spectrum, 

 and whose intensity is proportional to the energy content of the trans- 

 mitted light. For convenience I have used for the relative intensity 

 the same figures as those given in Table I for the transmitted energy. 



^ The measurements are given on p. 61 of a booklet — Wratten Light Filters — 

 published by The Eastman Kodak Company. 



