THE MICROSCOPE AND ITS PARTS. 27 



probably carry the object with it. If the microscope 

 is not inclined, the convex surface of the drop, and its 

 tremulous movements, will so affect the light that the 

 image will be distorted, and the observer will obtain 

 erroneous impressions. A piece of glass placed over 

 the water will flatten the surface, the distortion of the 

 image will be partly counteracted, and capillary 

 attraction will keep the liquid from entirely running 

 away. But ordinary glass is too thick for this purpose; 

 consequently thin glass prepared for microscopical use 

 must be purchased. This varies from No.i, measur- 

 ing about y-|-j- to ^-J-jj- inch in thickness, or thinner; No. 2, 

 about -j-^; and No. 3, from -^ to^^ inch. No. 2 glass 

 will beiithe 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 examination of an ob- 

 ject 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 this thin glass is an import- 

 ant 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. i glass in use for three months contin- 

 uously, frequently removing 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 



