1885.] NEW-YORK MICROSCOPICAL SOCIETY. 35 



pictures at a plane considerably above the focal plane of the 

 smaller ones. From this fact it would appear thai these insects 

 are furnished with eyes of two varieties, corresponding to our 

 long-sight and short-sight spectacles ; in other words, telescopic 

 and microscopical eyes, the telescopic looking upward and for- 

 ward, and the microscopical downward. The economy of such 



an optical structure in a parasitic insect which seeks its prey at a 

 distance, is so obvious that I need not stop to explain it. 



For showing multiple images, the most perfect eyes that I have 

 yet found are those of Blatta orienialis, or the Cockroach. As 

 the eyes of this insect are quite brittle, only a small part of the 

 cornea can be pressed flat in one piece. Yet a piece large 

 enough to fill the field commanded by a half-inch objective and 

 a £ ocular can be cut out with the punch. The many advanta- 

 ges which it possesses, more than counterbalance its lack of 

 superficial extent. For the lenses are very transparent, and com- 

 paratively large, and, being set in a moderately hard framework, 

 do not separate so as to destroy the achromatic combination. 

 Nor do the lenses which make up each combination slip upon 

 one another when subjected to slight pressure, as do the lenses 

 in the eyes of most other insects except the Coleoptera. When 

 the lenses do thus slip upon one another, each separate eye shows 

 two or three imperfect images instead of a single good one. The 

 chief advantage, however, which the eyes of the Cockroach pos- 

 sess over all others is that they may be mounted in glycerine and 

 thus kept perfectly transparent without losing their properties as 

 lenses. 



The usual method of exhibiting the multiple images is to place 

 the mounted cornea of the compound eye upon the stage, and 

 focus the microscope so much above it as to show a clear circle 

 of light in each facet, Then, if 9.ny small object be placed be- 



