104 PRACTICAL MICROSCOPY. 



front of the system focusses from the object not being 

 considered at all. 



Gundlach, in a rather abstruse article in the 'American 

 Monthly Microscopical Journal,' ii., 1881, tells us that 

 "working distance" depends upon (i) the focal distance 

 (nominal, it is presumed), (2) the aperture, (3) the number 

 of lenses of which the objective is composed, (4) the pro- 

 portionate curves of the lenses, and (5) the thickness of 

 the lenses. 



Great working distance is valuable in an objective only 

 when circumstances demand it. Thus, for dissecting, or for 

 the examination of opaque objects, a certain amount of 

 distance is requisite for manipulation and illumination ; but 

 when an object has been prepared and mounted, no more 

 working distance is absolutely required than will admit of 

 the use of the thickest covering-glass, and of the examina- 

 tion of a moderate depth of object. 



High-angle objectives of low power and consequent 

 shorter working distance will define much better than the 

 smaller apertures, and there is sufficient working distance, 

 even for dissecting, with the i-inch of 25 air angle. 



There is some difficulty in selecting a i-inch objective. 

 A glass of 40 air angle possesses considerable working 

 distance, being a power well suited for dissections, used in 

 conjunction with the erector ; while a J-inch objective of 

 80 air-angle scarcely gives o 03 inch of working distance, 

 focussing closer to the object than an ordinary J-inch objec- 

 tive of 85. 



With the i-inch and all higher powers the working 

 distance is very small, so that often the microscopist is 

 precluded from using covering-glasses of the ordinary 

 thickness. This is the case with all extremely high aper- 

 tures used as dry objectives. Immersion objectives, for 

 the same degrees of amplification, afford much longer 



