THE MICROSCOPE. 



Fig. 25 shows a scale of the small insect called Podura 

 Plumbea, the common Skiptail, magnified about five hundred 

 times. To define the markings on this scale clearly is the high- 

 est test of a deep achromatic object-glass; and this drawing is 

 given rather to explain what the observer should 

 look for, than as a very correct representation. 

 Fig. 26 is a scale or feather of the Menelaus But- 

 terfly; Fig. 27 is the hair of a singular insect, 

 the Dermestes; and Fig. 28 is a longitudinal cut- 

 ting of fir, showing the circular glands on the 

 vessels which distinguish coniferous woods. 

 These latter objects may be seen by half -inch or 

 quarter-inch achromatic glasses. Opaque objects 

 are generally better exhibited by inch and two- 

 inch glasses, when a general view of them is re- 

 quired, and by higher powers when we wish to 

 examine their minute structure. In the latter 

 case the light must be obtained by condensing 

 lenses instead of the metallic specula. 

 Fig. 28. Although the reflecting microscope is now 



very little used, it may be expected that we 

 should mention it. In this instrument, at Fig. 29, the object 

 O is reflected by the inclined face of the mirror M, and the 

 rays are again reflected and converged by the ellipsoidal re- 

 flector B B, which eflects the 

 same purpose as the object-glass 

 of the compound microscope. 

 It forms an image which is not 

 susceptible of the over-correc- 

 tion as to color before described, 

 and which therefore becomes 

 colored in passing through the 

 eye-piece. This fact, and the 



loss of light by reflection, will probably always render the re- 

 flecting microscope inferior to the achromatic refracting. 



The solar microscope has been so nearly superseded by the 

 oxy-hydrogen, that a brief description of the latter must suf- 

 fice, particularly as their optical principles are similar. 

 The primary object in both is to throw an intense light upon 



Fig. 29. 



