22 



MICROSCOPE AND ACCESSORIES. 



[CH. I. 



With illuminating and penetrating power the equivalent focus of the 

 objective must be considered as well as the numerical aperture. With 

 objectives of the same equivalent focus, to double the N. A. is to in- 

 crease the illuminating power 4- fold but the penetrating power is halved. 



The numerical aperture remaining constant, the illuminating and 

 penetrating power vary directly as the square of the equivalent focus ; 

 thus a 4 mm. objective would give four times the illuminating and 

 penetrating power of a 2 mm. objective. 



Of course when equivalent focus and numerical aperture both differ 

 the problem becomes more complex. 



For a consideration of the aperture question, its history and signifi- 

 cance, see J. D. Cox, Proc. Amer. Micr. Soc, 1884, pp. 5-39; Jour. 

 Roy. Micr. Soc, 1881, pp. 303, 348, 365, 388; 1882, pp. 300, 460; 

 1883, p. 790; 1884, p. 20. Carpenter-Dallinger, Chapters II and V. 



THE OCULAR. 



I32 A Microscopic Ocular or Eye-Piece consists of one or more converging 

 lenses or lens systems, the combined action of which is, like that of a simple mi- 

 croscope, to magnify the real image formed by the objective. 



Fig. 30. Sectional view of a Huygenian ocular (Hg. ocu- 

 lar), to show the formation of the Eye-Point. 



Axis. Optic axis of the ocular. D. Diaphragm of the 

 ocular. E. L. Eye-lens. F. L. Field-lens. 



E. P. Eye-point. As seen in section, it appears something- 

 like an hour-glass. When seen as in looking into the ocu- 

 lar, i. e., in trajisection, it appears as a circle of light. It is 

 at the point zchere the most rays cross. 



Depending upon the relation and action of the different 

 lenses forming oculars, they are divided into twogreat groups, 

 negative and positive. 



\ 33. Negative Oculars are those in which the real, in- 

 verted image is formed within the ocular, the lower or field- 

 lens serving to collect the image- forming rays somewhat, so 

 Sj that the real image is smaller than as if the field lens were 

 I absent (Fig. 21). As the field-lens of the ocular aids in the 

 : S formation of the real image it is considered by some to form 

 ^ a part of the objective rather than of the ocular. The upper 

 or eye lens of the ocular magnifies the real image. 

 \ 34. Positive Oculars are those in which the real, inverted image of the objec- 

 tive is formed outside the ocular, and the entire system of ocular lenses magnifies 

 the real image like a simple microscope (Fig. 16). 



Positive and negative oculars may be readily distinguished, as a positive ocular 

 may be used as a simple microscope, while a negative ocular cannot be so used 

 when its field lens is in the natural position toward the object. By turning the 



