CLASSIFICATION OF ANIMAL PEEPARATIONS. 423 



specimen ; if, however, it be required to do so, the glycerine 

 will be foxuid the best fluid for the purpose. 



Scales of Insects. — These minute bodies, familiarly known 

 as down, were first discovered and described by Leeuwenhoek; 

 in more modern times, the lines on their siu-faces have served 

 as objects for testing the defining powers of single lenses, 

 doublets, and achromatic combinations. The scales having 

 the greatest number of lines in a given space, and, therefore 

 the most difficult to define, are accurately represented 

 in plates 7 and 8, and a fuU description of the same wiU be 

 found under the head of test objects. In order to examine 

 them to the greatest advantage, they should be mounted dry, 

 after the plans described at page 314, and exhibited by 

 figs. 212-13 ; the scales are readily removed from the wiag by 

 merely pressing the latter very gently upon an ordinary slide, 

 or upon a piece of thin glass, to which they wUl adhere firmly; 

 they may then be covered up and cemented in the manner 

 previously recommended for dry objects, or as shown by 

 figs. 212-13. The scales from the wings and bodies of the 

 following insects will exhibit many varieties of markings, all 

 of which may be employed as tests : — 



Aluoita hexadactyla, Podura plumbea, 



Curculio imperialis, Polyommatus Arion, 



Gnat, Acis, 



Hipparchia janira, Adonis, 



Lasiocampa quercus, Alexis, 



Lepisma saccharina, Argus, 



Morpho Menelaus, Argiolus, 



Papilio Paris, Tinea vestianella. 



Spiracles aiid Tracheae of Insects. — The method of preparing 

 the trachese and spiracles of insects has already been described 

 at pages 361-2; they may be examined in situ in many of 

 the parasitic insects, in others the aid of dissection is neces- 

 sary for their due display. It must be borne in mind, that if 

 they be mounted in balsam they show best when full of air. 

 The following larvas and perfect insects wiU exhibit the 

 trachese and spiracles in a very beautiful manner : — 



