The Microscope. 365 



handle the thinnest of glass, and to do it with apparent careless- 

 ness, but in reality with the skill born of practice. 



The glass is to be obtained from the dealers in microscopical 

 supplies, and comes in three thicknesses, being ordered accord- 

 ing to the number, the number rising as tlie thickness increases. 

 Number 1 is the thinnest, with the exception of that wliich is 

 specially measured and used for special purposes. The thick- 

 ness of No. 1 is from 1-150 to 1-200 inch; that of No. 2 from 

 1-100 to 1-150 ; and of No. 3, from 1-50 to 1-100. The thick- 

 ness varies within these limits in almost every lot sent out by 

 the dealers. No. 3, the thickest, should not be used for any 

 purpose. It is entirely too thick for any but opaque objects 

 to be examined with very low powers, yet I should recom- 

 mend the beginner not to buy it for any purpose. It is 

 much better to use one thickness for every object, so that all 

 the preparations in the cabinet may be examined with high 

 powers, if necessary. A mount prepared for a high power 

 objective is as useful for a low power, provided, of course, it 

 can be appropriately studied with that power. Some objects, 

 as the reader knows, can be examined with high powers only ; 

 but this does not invalidate the advice always to use one thick- 

 ness of cover glass, and to have it as thin as convenient. No. 

 2, from 1-100 to 1-150 inch, is the proper glass to select and to 

 use for most objects. The facility of handling and manipulat- 

 ing it is soon acquired, and the advantages to be gained are 

 important. No. 1, from 1-150 to 1-200, is too thin for ordinary 

 use. It is intended only for those very high powers employed 

 by microscopical experts in original investigations. 



The glass can be obtained in squares and circles, both kinds 

 being sold by the ounce or by the dozen pieces. For mounting- 

 purposes the circles are the best, because the finished slide pre- 

 sents a better appearance with a neatly formed disk in the cen- 

 tre than with a squai'e. This is, however, the least important 

 reason. Most mounts have a ring of cement placed around the 

 edge of the cover in order to keep the enclosed fluid and object 

 in place, and to prevent the evaporation of the mounting 

 medium. With the contrivance called a turn table, to be 

 referred to hereafter, a ring of cement can be neatly and rapidly 

 spun about the cover's mai'gin ; with a square cover the apply- 

 ing of the finishing cement is more difficult and the result 



