134 THE USE OF THE MICROSCOPE 



of lens paper just moistened with xylol and afterwards 

 with distilled water. This requisite cleaning, the necessity 

 for a correction collar, and the consequent unavoidable 

 measurement of cover-glasses, are the main causes of the 

 comparatively small use of water-immersion objectives. 

 Another reason why the water-immersion objective has 

 not yet been fully appreciated, is that its performance is 

 much improved by diaphragming the source of light focused 

 on the object, until the illuminated part is just equal to 

 (or less than) the field of view.^ (The improvement of an 

 oil-immersion objective by this is somewhat less.) A 

 water-immersion objective of 1.2 or 1.25 aperture should, 

 with a corrected condenser, resolve one of the medium 

 valves of Surirella gemma, mounted in hyrax, into (a sharp 

 net, or) sharp rows of dots, with an eyepiece of about 15 

 times. (The sharpness of the picture is the test; not the 

 mere resolution, which can be done with a lower aperture.) 



Another Advantage. — One other advantage of the use 

 of a water-immersion objective is that, with water immer- 

 sion for the condenser also, and the use of the Detto slide 

 bar instead of a mechanical stage, the slide can be floated 

 on a capillary layer of distilled water on the stage (and 

 kept free from immersion oil running over the lower edge) ; 

 which allows an almost frictionless motion to and fro when 

 searching preparations under large covers. The water- 

 immersion objective has also the considerable advantage, 

 in searching a shde, that the cover-glass can be wiped clean 

 with a touch of a folded piece of blotting paper; so that the 

 dry ''finder" objective gives good definition, which is not 

 marred by smears of oil on the cover, as it is when a high- 

 power oil objective has been used just before. 



If a 40-times dry objective with a correction collar is 

 also used with a 70 or 90 water-immersion objective, one 

 measuring of the cover is sufficient for both correction 

 collars to be set. 



Summary. — The oil-immersion objective is best for 

 the study of objects permeated with immersion oil or bal- 



^See Beck, "The Microscope," Pt. 2. 



