366 The Microscope. 



much less attractive in appearance. For temporary study of an 

 object, when it is placed on the slip with a drop of water and 

 studied, but not permanently mounted by the fastening down of 

 the cover, thin squares are much more convenient as they are 

 pleasanter to handle, easier to be cleaned after use, and rather 

 cheaper, although the question of cheapness in cover glass is a 

 small one, since it is all inexpensive, and fortunately so for the 

 microscopist. If covers were as costly as they are fragile we 

 should be in an unenviable position. 



The circles may be had in various sizes from 3-lGths of an 

 inch in diameter up to f inch, and larger if desired and ordered 

 to be so cut. But as a rule the diameter increases by six- 

 teenths of an inch. The smallest sizes are not adapted to gen- 

 eral use ; thej^ are intended for special purposes, when the 

 microscopist desires to mount a single very small object, a 

 single Diatom, for instance. The largest sizes are also designed 

 for special purposes, such as very broad sections of animal 

 tissues, or some other large object that cannot be conveniently 

 divided into smaller portions. The most acceptable size for gen- 

 eral mounting is f inch, or perliaps 11-16 ; but the reader should 

 remember that the larger the disk the more easily it is broken. 



Thin squares may be had from i to | inches square. The 

 smaller size is not to be commended. The | inch the reader 

 will find the appropriate one, either for permanent mounting or 

 for the study of an unmounted specimen that is not to be pre- 

 served in the cabinet. 



But thin covers also come from the dealers in any but a 

 clean condition. If they should be used in the state in which 

 they are bought the object would be obscured and the examina- 

 tion be made very unsatisfactory. They must be carefully 

 cleaned before they can be used. This is a process demanding 

 rather more care than the cleaning of slips. Yet all that is 

 needed is practice. The thinnest covers can be as easilj^ and as 

 successfully cleaned by rubbing them between two folds of a 

 clean and soft old cloth as can the ordinary slip. And usuall}'^ 

 this is all that they need. They are never excessively dirt3 when 

 the}' come from the dealer, and a gentle application of the soft 

 old rag iield between the thumb and finger is generally all Shat 

 is needed. About the only precaution to be remembered when 

 using this method is to have the cloth perfectly smooth. If it 



