BUTTERFLIES OF THE DISTRICT OF COLUMBIA 241 



The strips of quartz obliterated the portions of the wings beneath 

 them as completely as did the glass. 



Experiments with various kinds of films gave divergent, though 

 always unsatisfactory, results. The color values were always the 

 same with films as with the plates. 



If the phenomenon were due to light of a very short wave length — 

 that is, if it were a sort of invisible phosphorescence — this light 

 should pass through the quartz as readily as through the cellophane. 

 But the quartz completely obliterates those portions of the wings 

 over which it lies even in exposures of 30 days' duration. 



That the phenomenon is due to the effect upon the photographic 

 plate of some gas and not to light was from the first maintained by 

 Dr. Raymond S. Davis, of the Bureau of Standards, and by Dr. A. J. 

 Olmsted, of the National Museum, and their assumption appears to 

 have been shown to be correct by these experiments. For while gas 

 as well as light will pass through cellophane, glass and quartz are 

 impervious to gases. 



It was suggested by Dr. Burt H. Carroll, of the Bureau of Stand- 

 ards, that possibly the effect was due to sulphur. As a result of 

 this suggestion exposures were made on burnished metallic silver. 



Although the butterflies used for these exposures had been dead 

 for about two months, faint images were produced on the silver by 

 an exposure of 24 hours. The images, though recognizable, were so 

 faint as to appear as a slight colorless frosting, and while presumably 

 due to some gas containing sulphur, there is no conclusive proof 

 of this. 



So far as we are able to judge from the facts as present available, 

 the images produced on photographic plates by direct contact with 

 butterflies' wings are due to the slow decomposition of the pigments 

 in the wings and the resulting emanation of some gas or gases con- 

 taining sulphur. But fully conclusive and convincing proof of the 

 cause of the phenomenon is not yet at hand. 



