646 



SUMMARY OF CUEEENT RESEARCHES RELATING TO 



almost upright, and soon evaporates. Oil requires great care in manipula- 

 tion and loss of time in cleaning off after use. Glycerin is free from these 

 objections. It will remain three or four days limpid and free from evapora- 

 tion, and only requires cleaning off with a camel-hair pencil dipped in 

 water, and the lens dried with blotting-paper. This objective has more 

 brilliant definition, deeper penetration, and a greater working distance from 

 the object than any others of its class at much higher prices." 



It is not a little surprising that in these days an optician should show 

 such a want of appreciation of elementary optical principles. Glycerin 

 having a lower refractive index than the oil used for immersion the objective 

 is not a homogeneous-immersion objective, with which, therefore, it cannot 

 be compared. Glycerin having a lower refractive index than the fluid used 

 for homogeneous-immersion, the aperture of glycerin objectives, and with it 

 the brilliancy of the definition, is necessarily reduced. The " deeper 

 penetration" is of course simply a function of the reduced aperture. Why 

 a glycerin objective should have a greater working distance it would puzzle 

 an optician to say. 



Apart from optical errors, it is equally erroneous to say that glycerin 

 requires less care in manipulation and takes less time to clean off than oil, 

 while its well-known tendency to absorb moisture, and therefore to change in 

 index, is more than a compensation for its alleged freedom from evaporation. 

 It will be news to many that " water necessitates the Microscope being used 

 nearly upright." 



Notwithstanding the glowing panegyric on this objective the notice of its 

 virtues, although stating that it is a 1/16 in., omits any mention of its aperture. 



Zeiss's Objective-cliang^er, with slide and centering- adjustment. — 

 This contrivance (figs. 165 and 166) is designed to provide (1) accurate 



Fig. 165. 



Fro. 166. 



centering, and (2) rapid change of the objectives, 

 parts, the tube-slide and the objective-slide. 



It consists of two 



