CHAPTER III 



PHYSICAL NATUEE OF ANIMAL LIGHT 



Interest in the light of animals from a physical stand- 

 point has centred around questions of quality, efficiency 

 and intensity, but in only one group of luminous animals, 

 the beetles, have accurate measurements of these charac- 

 teristics been made. This is due in part to the abundance 

 of these forms and their appeal to human interest and in 

 part because they are among the brightest of luminous or- 

 ganisms. Weak lights are not only difficult to measure but, 

 when dispersed to form spectra, give bands so faint that 

 their limits are very difficult to see and more so to photo- 

 graph. Very few organisms produce light visible to the 

 fully light-adapted eye. Although their light may seem 

 quite bright to the dark-adapted eye, the dark-adapted eye 

 is a poor judge of the quality, i.e., the color of a light. 

 This is because of the Purkinje phenomenon, a change in 

 the region of maximum sensibility of the retina with 

 change in intensity of the light. For an equal energy 

 spectrum, to the normal, completely light-adapted eye, 

 yellow-green light of wave-leng^th, A = .565/*, appears thfe 

 brightest, but when the light is made fainter the maximum 

 shifts first to the green and then to the blue. The dark- 

 adapted eye can see green or blue better than yellow and 

 for this reason weak lights will appear more green or 

 blue than stronger ones of the same energy distribution. 

 Also two weak lights of the same spectral composition 

 may appear different in color if they differ much in inten- 

 sity. This is illustrated in Fig. 6. 



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