THE MICROSCOPE. 5 



mounted dry on a black disc exhibit their form and mark- 

 ings, as if the more elaborate parabolic reflector was beneath. 

 To test this, obtain a slide of Heliopelta, or of Tsthmia enervis, 

 or of Arachnoidiscus, which will show the advantage of the 

 binocular with a very moderate power. 



Fossil foraminated shells (Barbadoes) . 



Spicules of gorgonia. 



Section of Kush. 



Capsules of Moss. 



Any of these will answer the same purpose. 



Whilst adding a few thoughts to this introductory chap- 

 ter, let me answer one or two questions not unfrequently 

 put to me. 



How much does this microscope magnify ? 



That is, a small binocular with simple eye-piece, and three 

 object-glasses of two-inch, one-inch, and half-inch focus. 

 The scale of magnifying power varies with different makers; 

 but if I give my own microscope as an example, it will help 

 to make others understood. 



The two-inch object-glass, which has an angular aperture 

 of 15, will magnify twenty-five diameters, or 400 times 

 superficial measurement. 



The one-inch object-glass has usually about 23 angular 

 aperture, magnifies fifty to sixty diameters, or 960 times 

 superficial measurement. 



The half-inch object-glass has an angular aperture of 60 

 or 65, and consequently magnifies 120 diameters, or 1920 

 times superficial measurement. 



Again, 1 am asked, What is the angle of aperture ? 



This cannot here be explained beyond the brief state- 

 ment, that it is the angle made ly two lines from opposite 

 sides of the aperture of the object-glass with the point of the 

 focus of the lens. A diagram is necessary to make this 

 quite clear. 



The two-inch object-glass requires a certain distance, as 

 at A, to bring the object under examination into distinct- 

 ness or focus. 



The inch, half-inch, quarter-inch glasses, require nearer 

 and nearer approach to the object for the same purpose. 



