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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



ing graph, Figure 1. The spectral regions used are indicated by the 

 transmission curves. The wave lengths are plotted as abscissae and 

 the jDercentages of light transmitted by the filters as ordinates. The 

 continuous curves indicate the regions of transmission for each of 

 the filters; the blue filter (B) transmitting the region between 4,000 

 and 5,000 A units, the green filter (G) transmitting between 4,800 

 and 5,900A units, the yellow filter (Y) transmitting all visible 

 wave lengths longer than 5,300 A, and the red filter (R) transmit- 

 ting all wave lengths longer than 5,900 xV. For the sake of convenience 

 the observable response curves have been plotted upon the same dia- 

 gram in dotted lines. Tlie response in the red was found to be zero. 

 The response to yellow light has been arbitrarily assigned the value 

 " unity." Using a logarithmic scale (inside the frame) the relative 

 responses in green and blue have been indicated. In the right-hand 



4000 5000 eOOO 7000 



Wave length 

 Figure 1. — Phototroplsm by filter method 



rhototropic sensitivity. 



Transmission of filters. 



curve each point is i)lottcd at the wave-length center of gravity of 

 the region for each filter, in the case of yellow, only counting those 

 wave lengths not included by the red filter. 



This curve plotted through these three points may be regarded as 

 a first approximation. On the basis of this curve the centers of 

 gravity were redetermined where each wave length was weighted 

 according to resi)onses as indicated by the first approximation curve. 

 The middle curve was thus obtained by simply shifting the points 

 to the weighed center of gravity wave lengths. Using this second 

 approximation curve as the basis for again reweighting, the third 

 or left-hand curve was obtained. Reweighting was, of course, im- 

 possible for the blue region, as data are not available on the shorter 

 wave-length side. 



These results are presented for the sake of comparison with the 

 results obtained in the more elaborate experiment carried out with 



