494 



THE MICROSCOPE. 



dinal and trunsverse striae, which should be clearly defined, 

 and the spots separated : this is a good test of the denning 

 power of a quarter-inch object-glass. 



No. 3, HipparcMa janira, Common Meadow Brown 

 Butterfly scale : on this we see a number of brown spots 

 of irregular shape and longitudinal striae. 



No. 4, Pontia brassica, Cabbage-butterfly, affords an 

 excellent criterion of the penetration and definition of a 

 microscope: it is provided at its free extremity with a 

 brush-like appendage. With a high power, the longitu- 

 dinal markings appear like rows of little beads. Cheva- 

 lier's test-object is the scale of the Pontia brassica, the 

 granules of which must be rendered distinct. Mohl and 

 Schacht use Hipparckia janira as a test for penetration, 

 with a moderate angular aperture and oblique illumina- 

 tion. Amici's test-object is Navicula Rhomboides, the dis- 

 play of the lines forming the test; this is also a good test 

 for angular aperture. 



Fig. 249. Portions of Scales, magnified 500 diameters. 



la, Portion of Scale of Morpho Menelaus. 5a, Portion of Large Scale of Podura 

 Plumbea. 7, Scale from the Wing of Gnat; its two layers are here represented. 

 8, Portion of a Large Scale of Lepisma Saccharina. 



The Tinea vestianella, Clothes-moth, possesses very deli- 

 cate and unique scales ; two of these are imperfectly 

 represented near the Acarus taken from one of these moths, 

 at page 461. The feathers from tho under-side of the 



