80 THE MICROSCOPE. 



aperture will transmit them all. With a minute 

 object, however, the diffracted rays are widely spread 

 out so that a small aperture can admit only a fractional 

 part to admit the whole or a very large part, and 

 consequently to see the minute structure of the object, 

 or to see it truly, a large aperture is necessary, and in 

 this lies the value of aperture and of a wide-angled 

 immersion-objective for the observation of minute 

 structures. 



Object-glasses. With regard to the selection of 

 object-glasses, this will depend on the work in which we 

 may be about to engage. The amateur or student will 

 be well advised to commence with moderate or even 

 low powers, as a 3-inch, a 2-inch, a 1-inch, and a 

 f -inch focus. These powers used with the A eye-piece 

 will give a range of magnification of from 20 to 70 

 diameters ; and with the B eye-piece will be increased 

 to 120 diameters. Zeiss, of Jena, has lately constructed 

 a very useful adjustable objective; by an ingenious 

 screw-collar arrangement the relative position of the 

 front lens is changed, and a range of power, varying 

 from 12 to 24, or 30 diameters, is obtained. This 

 object-glass consists of a convex back lens and a con- 

 cave front lens (both achromatic), the distance of 

 which is changed by means of a screw acting in the 

 manner of a correcting collar of wide range. When 

 the collar is put to 10, the objective has its minimum 

 focal length or maximum power, approximately corre- 

 sponding to that of a single 2-inch lens. By closing 

 the collar the equivalent focal length increases, the 

 back lens is made to approach nearer the eye-piece, 

 and the magnifying power is varied, so that when the 

 collar is put to the actual power of the objective 

 corresponds to that of a 4-inch lens. By a judicious 

 use of the draw-tube of the microscope a further mag- 

 nification of the image can be obtained, which is of 

 value if botanical sections, opaque objects, and whole 

 insects are under examination. With an erecting eye- 

 piece the lower powers above mentioned are most useful 

 for dissection purposes. 



Objectives of medium power are the |-inch, 4-10ths, 



