134 THE MICROSCOPE. Sept. 



closing the object, the movements of the stage then 

 bringing the ring indistinctly into view, and the project- 

 ing borders of the square protecting both the immersion 

 fluid above it, and the object beneath. 



The front lens of these objectives must also be care- 

 fully cleaned and dried after the immersion-fluid has 

 been as carefully applied. But the advantages obtain- 

 able more than counterbalance the inconveniences. 



The fluid is always applied between the lens and the 

 cover-glass. It is difficult to imagine any human being 

 endowed with such unmitigated stupidity, that he should 

 pour the immersion-liquid into the tube of the lens- 

 mounting, yet instances of the kind have been reported. 



It is recommended by some that the water, glycerine, 

 oil or other fluid be applied in a small drop to the cover, 

 and the objective racked down until the front lens comes 

 in contact with it. The only advantage of this method, 

 and that advantage is very slight, is that by it the prob- 

 ability of disarranging the object by the pressure of the 

 thick liquid compressed between the cover and the lens, 

 is lessened ; and the reader may prefer this method, es- 

 pecially when using glycerine-immersion or homogene- 

 ous-immersion objectives, but a great disadvantage is 

 that the moment the objective touches the liquid, the 

 microscopist loses the power to appreciate the distance 

 between the lens and the cover, and is therefore Ifkely 

 to rack down too far and so to do some damage, or not 

 far enough and thus leave too much to be done by the 

 fine-adjustment screw. 



When using glycerine or homogeneous-fluid, I am in 

 the habit of applying a drop to the front lens of the ob- 

 jective instead of to the cover-glass. This may be done 

 by means of the cork from the bottle of fluid, a drop be- 

 ing allowed to form at one edge, whence it is carefully 

 placed on the objective without touching the cork to the 

 lens-front ; or a rod may be forced through the cork and 



