MICROSCOPE AND ACCESSORIES. 



\_CH. I. 



is N. A. = n sin u ; N. A: representing numerical aperture, n the in- 

 dex of refraction of the substance in front of the objective, and u the 

 semi-angle of aperture. 



For example, take three objectives each of 3 mm. equivalent focus, 

 one being a dry, one a water immersion, and one a homogeneous im- 

 mersion. Suppose that the dry objective has an angular aperture of 

 106 , the water immersion of 94 and the homogeneous immersion of 

 90°. Simply compared as to their angular aperture, without regard to 

 the medium in front of the objective, it would look as if the dry objec- 

 tive would actually take in and transmit a wider pencil of light than 

 either of the others. However, if the medium in front of the objective 



Figs. 27-29 are somewhat modified from 

 Ellenberger, and are introduced to illustrate 

 the relative amount oj 'utilized light, with dry, 

 water immersion and homogeneous immer- 



27 sion objectives of the same equivalent focus. 

 The point from which the rays emenate is in 

 air in each case. If Canada balsam were in 

 place of the air beneath the cover glass there 

 would be practically no refraction of the 

 rays on entering the cover glass (J 16). 



Fig. 27. Showing the course of the rays 

 passing through a cover glass from an axial 



28 point of the object, and the number that final- 

 ly enter the front of a dry objective. 



Fig. 28. Rays from the axial point of the 

 object traversing a cover of the same thick- 

 ness as in Fig. 26, and entering the front lens 

 of a water immersion objective. 



Fig. 29. Rays from an axial point of the 



29 object traversing a cover-glass and entering 

 the front of a homogeneous immersion objec- 

 tive. 



is considered, that is to say, if the numerical instead of the angular ap- 

 ertures are compared, the results would be as follows : Numerical Aper- 

 ture of a dry objective of 106 , N. A. = n sin u. In the case of dry ob- 

 jectives the medium in front of the objective being air, the index of 

 refraction is unity, whence n = 1. Half the angular aperture is 1 f-° = 

 53 . By consulting a table of natural sines it will be found that the 

 sine of 53° is 0.799, whence N. A. = n or 1 x sin u or 0.799 = 0.799. 



With the water immersion objective in the same way N. A. = n sin u. 

 In this case the medium in front of the objective is water, and its index 

 of refraction is 1.33, whence n = 1.33. Half the angular aperture is 

 -4° = 47°, and by consulting a table of natural sines, the sine of 47 is 



