274 The Microscope. 



glass in sulphuric acid for a short time, adding water carefully, 

 so as not to generate suddenly too much heat. When the liquid 

 is nearly neutral, the vessel is placed under the hydrant and a 

 stream sent among the covers. They are then left in the water 

 until the owner is ready to take them out and wipe them dry. 

 Tiiis troublesome process is seldom necessary, and, indeed, the 

 use of sulphuric or of other strong mineral acid, would better be 

 avoided by the novice unless he knows something of chemistry. 

 Cleaning may be accomplished by gently wiping the glass with 

 a soft cloth, the cover being held between the thumb and finger 

 of the left hand, while the thumb and finger of the right apply 

 the cloth to both surfaces at once. Considerable skill and 

 practice are required to do this successfully, for the glass is very 

 thin and very brittle, but after a few attempts covers will seldom 

 be broken. The cloth should be carefully kept smooth, so as to 

 avoid the irregular pressure of wrinkles. 



Several mechanical cleaning devices have been described, but 

 nothing is better than two smooth wooden blocks, each with a 

 surface tightly and smoothly covered with soft, thick chamois 

 skin. The cover is placed on one block while the other rubs its 

 surface until it is clean, when the whole is turned over, and the 

 other block rubs the other side of the glass. With this simple 

 contrivance it is hardly possible to break the thinnest cover. 



After being cleaned the glass sliould be kept clean, as well as 

 free from scratches. It is exposed to the latter if thrown loosely 

 into a box, where a particle of hard dust may do mischief. Mr. 

 C. E. Hanaman, of Troy, N. Y., has described a method of keep- 

 ing the covers protected from accident and dirt. He places 

 them in drawers or boxes filled with narrow strips of new white 

 blotting paper, between which they are stood on edge. This 

 method, Mr. Hanaman says, not only preserves the covers from 

 breakage and enables him readily to pick them out when wanted 

 for use, but also assists him to select for special preparations 

 those of the most desirable thickness, for, by holding the drawer 

 or box between the eye and the light, it is easy to select the 

 thickest or the thinnest. 



The glass varies a good deal in thickness even in the same lot 

 as supplied by the optician. Its thickness may be most con- 

 veniently measured by some sort of micrometer gauge, of which 

 there are several in the market for the use of machinists. Pr 



