THE MICROSCOPE AND ITS PARTS. 29 



tirely running away. But ordinary glass is too thick 

 for this purpose, consequently thin glass prepared for 

 microscopical use must be purchased. This varies in 

 thickness from No. 1, measuring about r ^ to ^ihr inch 

 or thinner ; No. 2, about T ^ ; and 'No. 3, from ^V to T V 

 inch. ~No. 2 glass will be the proper thickness. It can 

 be obtained either in circles of various sizes or in 

 squares. For permanent mounts the circles are usually 

 employed. For temporary purposes, for the examina- 

 tion of an object that is not to be preserved for future 

 use, or when many examinations of separated parts of 

 the same large specimen are to be made, the writer much 

 prefers thin squares, and always uses them. They are 

 pleasanter to handle, they are more easily wiped dry 

 and with less liability to breakage, and their cost is 

 somewhat less than circles of the same thickness. 



The matter of cleaning thin glass is an important one, 

 and unless the " knack " is soon learned, the beginner 

 will be surprised at the rapidity with which his covers 

 will disappear. This skill, however, is readily attained. 

 The writer has had the same thin square of No. 1 glass 

 in use for three months continuously, frequently remov- 

 ing and reapplying it during the five or six hours of 

 daily evening work in which it did important service, 

 and in the end he became quite attached to it as to a 

 good friend. But a hasty move while cleaning it, or a 

 little undue pressure, finally sent it on the way that -thin 

 covers often travel. To clean without much risk of 

 breaking, take the square with two opposite edges, that 



