368 WILLIAM L. DOLLEY, JR. 



As already stated, three methods have heretofore been used 

 to prevent the functioning of one eye; extirpation, searing with 

 a hot needle, and covering with asphalt varnish. The latter 

 method was used exclusively in the present work, because it was 

 believed that fewer disturbing factors would be introduced 

 thereby. 



In the early part of the work, one eye was covered with one 

 or two coats of the asphalt varnish. After having made some 

 experiments with animals treated thus, it was found, to my sur- 

 prise, that insects with both eyes covered in this way still oriented 

 fairly precisely, arid went toward the source of light. Thus it 

 is evident that the varnish as used did not exclude all of the 

 light. The eyes were then painted repeatedly until the coats 

 were so thick as to be distinctly evident when the observer was 

 several feet from the butterflies. Under these conditions the 

 animals were indifferent to light. Warned by this experience, 

 the blinded insects used in all future experiments were so treated 

 that it seemed certain that the eye was in every case effectively 

 covered. Moreover frequent examinations were made to make 

 sure that the varnish had not cracked or fallen off; and new 

 coats were from time to time applied to make assurance doubly 

 sure. 



In work of this sort it is important that the varnish be of such 

 nature that it does not injure the eye in any way. The effect 

 of the covering was consequently repeatedly tested by removing 

 it from the eyes after it had been on for some time. Insects 

 thus tested behaved as did those whose eyes had not been 

 blackened, showing no effect from the varnish. 



The supply stock of butterflies was ordinarily kept in a large 

 cage which was four feet high, four feet long and two feet wide. 

 This was fastened against a south window in such a way that 

 the window formed one side of the enclosure. The opposite side 

 was also of glass and faced a small laboratory room. The other 

 two sides, the top, and the bottom, were of wood. Careful 

 observations were made on the insects in this enclosure, from 

 time to time, throughout the whole period over which the experi- 

 ments extended. But a much more thorough investigation of 



