THE OBJECTIVE. 59 



which affords the observer a deeper insight into struc- 

 ture. The objective having the longest working dis- 

 tance, as a rule, possesses the greatest amount of pene- 

 tration. Theoretically, according to Professor Abbe, the 

 penetration of an objective decreases as the square of 

 the angular aperture increases. The botanist or phy- 

 siologist, studying the minute anatomy of plant or 

 animal, would gain a very imperfect idea of the 

 structural elements entering into either, unless 

 the objective possessed good penetration. It is, how- 

 ever, somewhat unusual to find good penetrating or 

 separating power combined with equally good definition 

 in any objective. The latter quality is compatible only 

 with the highest attainable aperture. 



Fio. 35. Wenham's Binocular Objective. 



Penetration is an indispensable quality for the bino- 

 cular microscope, consequently opticians have been 

 induced to furnish special forms of object-glasses for 

 use with this form of instrument. Mr. Wenham carried 

 a kind of speciality into the construction of high-power 

 objectives for the binocular, by mounting a prism in a 

 separate tube, and slipping it down the objective, 

 and letting it almost touch the back lens. Fig. 35 

 represents a fth objective of the kind, full size, 

 with correcting adjustment. D being the objective 

 complete, and the tube with prism fixed in its place. 

 The objective, it will be seen, is shorter than an ordinary 

 Jth, and can be made to answer a double purpose. ^ It 

 becomes more effective as a homogeneous-immersion 



