On Mounting. 45 



ON MEASURING COVEK GLASSES. 



For ordinary students' work the No. 1 cover glasses 

 will do perfectly well, as they are very near the thick- 

 ness to which object glasses, without a correction collar, 

 are adjusted ; but when high power glasses are to be 

 used, it facilitates the work very much, to know the 

 thickness of the cover glass under which the specimen 

 is mounted, and with very high powers, or those with 

 wide angle of aperture, the cover must be at least '004 

 to enable the glass to work through it. Powell and 

 Lealand's -^ water immersion requires a cover glass 

 003 of an inch. 



To save expense, as dealers charge a high price for 

 very thin measured glass, the student will find it a 

 great advantage to purchase a small American steel 

 gauge, sold by Buck, Holborn ; it is made for measur- 

 ing the thickness of sheet brass in fine work, but does 

 admirably for measuring cover glasses ; the way to use 

 it will be at once apparent. With this instrument let 

 the student measure an ounce of No. 1 glass, and select 

 all those that are '004 and under, they may be put 

 away for future use. This is not much trouble, and a 

 few of these very thin glasses will last a long time, as it 

 is only in special work that they will be required. The 

 oil immersion lenses have also done away in a great 

 measure with the necessity of using measured cover 

 glasses. 



ON CLEANING COVEE GLASSES. 



The following plan will be found a very good one, 

 both in the saving of time and breakage. 



