io6 ELEMENTARY TECHNOLOGY. 



of the diameter to the focus of the lens. The larger angle 

 gives better definition of objects in the focal plane, but moder- 

 ate angles give more penetration, or define to some extent, 

 below the exact plane. 



Numerical aperture is a standard of comparison with all 

 kinds of objectives, and gives the light-grasping power of the 

 entire combination. For instance, if a dry objective couldtake 

 in all the rays of the maximum air angle, or 180, a water 

 immersion of N A of 96, or an oil or homogeneous immer- 

 sion of 82, would be equivalent. 



The unit of N A is the maximum air angle of i8o = N A 



1. 00. 



To illustrate, suppose an objective of 60 air angle (.5 N A) 

 transmits a circle of rays of ^ inch diameter, then one of 97 

 (N A .75) will transmit one equal to ^ inch diameter, one of 

 1 80 = 96 water angle, or 82 oil angle (N A i.oo; will ^ 

 inch diameter, 180 water angle = N A 1.33, transmits a circle 

 of y% inch diameter, and 180 oil angle (N A 1.52) =2^ inch 

 diameter. 



II. THE COMPOUND MICROSCOPE. 



i . General view. 



In its simplest form the compound microscope consists of 

 two convex lenses, Fig. 8, PI. I, A the eye glass or ocular, B 

 the object glass or objective. The rays from an ob- 

 ject, O, in the focus of B, form a large but inverted image 

 at a distance on the other side, which is again magnified 

 by A. The magnifying power of a compound micro- 

 scope may be increased by lengthening the distance be- 

 tween the objective and the eye glass, but since there are 

 limits in practice to this mode, a stronger eye glass may be 

 used with the same length of tube. Since magnification 

 brings every error into prominence, it is also best for increased 

 power to use more powerful and better corrected objectives. 

 On account of the divergence of the rays from the objective, 

 a collecting lens, C, called a field glass, is interposed between 

 A and B. This is usually combined with A and called the 

 eye piece. 



