ON CUTTING SECTIONS OF THE EYES OF INSECTS, &C. 5 



whatever instrument may be used for cutting ; and finally, of 

 the instrument I have devised for this purpose. 



The preparation for cutting is to the last degree important, for, 

 however perfect the cutting instrument may be, if the head be not 

 properly prepared, all attemps at sectioning it will be in vain 

 I have found, after many trials, that alcohol, 60° over proof 

 gives the most uniform results : but previous to putting the insect 

 into spirits I give it a gentle shaking in a phial of Benzine. I 

 do so because, in the subsequent embedding in bees-wax, greasiness 

 would prove fatal to the adhesion of the wax. Now oil dissolves 

 wax, with which it mixes mechanically, but alcohol effects the 

 complete separation of its components, so that alcohol as a 

 hardening medium has here a great advantage, there being an 

 affinity at the temperature necessary to melt the wax, between it 

 and that substance. The head may be allowed to remain in spirits 

 from four to forty-eight hours. I have cut some very good 

 sections after four hours' soaking, while others, after forty-eight 

 hours' preparation, were still soft ; again others, after forty-eight 

 hours' soaking were absolutely brittle. This appears to indicate 

 that some condition of which we can only guess the nature, 

 influences the result. 



The great difference inter se in the composition of the layers 

 of the eye, and the different extent to which they are influenced 

 by alcohol, constitutes to my mind the difficulty of preparation. 



I have therefore thought that the best heads on which to operate, 

 would be those of insects just on the point of emergence from the 

 chrysalis or pupa state, as the mass of the head is then more 

 nearly homogeneous, the integuments not having been hardened 

 by exposure. 



The head having been hardened, the next step is to imbed it 

 in wax or some other more suitable material, to hold it firmly 

 . during the operation of cutting. For this purpose I have used 

 , unbleached bees-wax, which I abandoned for a wax of my own 

 preparation ; I did so, because I found that the melting point of 

 the wax (142° F.) was dangerously near that at which complete 

 collapse of the head might be expected — if the cornea be at all 

 soft, as in the case of dipterous insects. Some use solid pa- 

 raffin, which melts at the temperature of 128° F. This is certainly 

 a safer medium, but it is to my mind rather brittle ; I therefore 

 sought for some wax which would have a low melting point, and 



