52 THE NATURE OF ANIMAL LIGHT 



Ives investigated the question further by the phosphor- 

 photographic method. ' ' In brief it consists of this : Phos- 

 phorescence, which is excited in various substances by 

 exposure to short waves (blue, violet or ultra-violet), is 

 destroyed by exposure to longer waves (orange, red, infra- 

 red). Thus, a surface of Balmain's paint or of Sidot 

 blende, excited to phosphorescence and then exposed in a 

 spectrograph, will have areas of reduced brightness wher- 

 ever long-wave energy has fallen upon it. If this surface 

 is then laid on a photographic plate for a short period, 

 a permanent record is obtained on the plate after develop- 

 ment." Preliminary tests showed that the method was 

 applicable in the case of weak light such as the firefly 

 spectrum and also if the light is intermittent like the fire- 

 fly. With Sidot blend (ZnS) the extinguishing action ex- 

 tends from A = 0.6/A to X = 1.5/ji. A sheet of deep ruby 

 glass, which cut off all the visible rays of the firefly but 

 allowed infra-red to pass, was placed between the firefly 

 light and a surface of phosphorescent Sidot blend which 

 was exposed to the firefly flashes for three and a half 

 hours. No extinction of phosphorescence occurred, while 

 without the ruby glass, extinction, due to the orange rays 

 of the visible firefly light was noticeable in 20 minutes. 

 There is thus no infra-red of an intensity at all comparable 

 to the visible as far as A. = 1.5/x, the lower limit of the 

 phosphor-photographic method. Coblentz (1912) had ex- 

 amined the transparency of the dry chitinous integument 

 of various fireflies (Fig. 10) in the infra-red and reports 

 it to be fairly transparent down to A, ^ 2.8/x, opaque be- 

 tween A = 2.8/i and A =i 3.8/^, transparent again to A = 6j«, 

 and opaque beyond that. The infra-red could, then, if it 

 were emitted, largely pass through the integument which 



