60 MICROSCOPIC OBJECTS. 



The scales of the eel when mounted in Canada balsam 

 are almost obliterated under common hght, but with 

 polarised light are brilliant objects. 



Scales of Insects. — Butterflies, moths, and many other 

 insects, are covered with scales or feathers, overlapping- 

 each other like the tiles on the roof of a house. They 

 vary greatly in form and size, and from the difficulty 

 with which the structure of many of them is developed 

 by the microscope they become excellent tests of its 

 penetrating power. (See Test Objects). In the Catalogue 

 are introduced the names of those insects which possess 

 the most interesting and peculiar scales. Those taken 

 from the red parts of the wing are not so various as the 

 scales from the white and blue colours. 



Scales of Euploea limniacea (?). — Some observers un- 

 acquainted with the Hned test-objects experience a diffi- 

 culty in bringing out the cross strice on the scales of the 

 Morpho Menelaus and others more deHcate. It may be 

 useful to mention, therefore, for this purpose, the scales 

 from the wing of the Euploea limniacea, and the blue ones 

 from the Papilio Paris, as the cross striae on these are 

 easily developed under a power of 100 diameters and 

 oblique illumination. 



Shell. — ^The organic structure of shell has long been 

 a subject of discussion among scientific men. Some have 

 considered it as produced in alternate layers of animal 

 and earthy matter, the one secreted m the day and the 

 other at night. Again, others consider the shell of a 

 cellular nature, in which after its formation earthy matter 



