40 MICROSCOPIC OBJECTS. 



in two groups, each composed of several distinct eyes, of 

 quadrangular, hexangular, circular and other forms, each 

 eye consisting of several layers of meniscus lenses, which 

 are easily separated by maceration, and their focus mea- 

 sured in the same manner as minute lenses. If a clus- 

 ter of the eyes be placed under the microscope, at a dis- 

 tance without its focus equal to their focal length, the 

 lens of each eye will exhibit a distinct image of a candle 

 flame, or other object placed before it, in the same man- 

 ner as a telescope would do ; indeed the whole becomes 

 4n fact an inverted telescope. To observe this effect the 

 microscope should be horizontal, and no mirror or con- 

 denser should be used behind the object. 



The number of eyes in a single group often amounts 

 to thousands. The eyes of spiders are single; they have 

 from four to twelve, variously arranged ; those most par- 

 ticularly worthy of notice are given in the list. In addi- , 

 tion to compound eyes, some insects have in front of 

 their head three distinct single eyes. 



Tlie eyes of the Harlequin Beetle are best suited for 

 the oxyhydrogen microscope, being large and well sepa- 

 rated from each other. The eyes of the Crawfish are 

 square, and show their optical character very readily. 



Feet, Insects. — The structure of the feet of those in- 

 sects which support themselves on polished surfaces, and 

 against the force of gravity, is worthy of microscopic 

 examination, and, like that of their eyes, is as yet but 

 imperfectly understood. The viewing them as opaque 

 objects, combined with observations on other specimens 



