244 The Microscope. 



cement or metal cell, and the thin glass cover applied to keep 

 out the dust, and often to hold the specimen in position. The 

 aft of mounting objects for microscopical examination is an at- 

 tractive one, demanding much good judgment, care, skill, neat- 

 ness, and delicacy of touch. A neatly finished slide is often a 

 beautiful thing, aside from the beauties of the included object 

 visible to the microscope alone, but it is in no way essential to 

 the study of the specimen. The microscopist, oftener than not^ 

 mounts his objects temporarily, using a drop of water or of gly- 

 cerine, allowing the thin cover to cling by capillary attraction,, 

 removing it and "cleaning up" when the examination is finished. 

 Some objects cannot be permanently preserved. It is then neces- 

 sary to study them as temporary mounts or not at all. Living 

 creatures, whose life-history or the action of whose organs is to 

 be observed, must be arranged in some way that shall not inter- 

 fere with their freedom, except so far as is necessary to restrict 

 them within a limited space. 



The thin glass prepared to prevent evaporation or as a protec- 

 tion to the objective or to the object, is an impoitant substance 

 to the microscopist, since it is used for all the foregoing pur- 

 poses, and for more. Before it was obtainable as easily and 

 cheaply as at present, microscopists used very thin pieces of 

 mica, and for use with exceedingly high power objectives whose 

 working distance is excessively short it is to a certain extent 

 still used. In this* case it is advantageous, as it is not easily 

 broken, and since its flexibility protects the front lens of these 

 very costly objectives. For ordinary purposes, however, thin 

 glass is employed and is preferable. The older microscopists 

 preserved their objects between two pieces of window glass. 



The method of manufacturing this thin microscopical glass is 

 a secret known only, I believe, to the Messrs Chance, of Birm- 

 ingham, England. Some time ago the report passed the rounds 

 of the journals to the efiect that a method of making it had 

 been discovered in Germany, and that it would soon be supplied 

 very cheaply, but nothing further has been heard from itff late. 

 All that is used in this country is imported in sheets, and cut by 

 the dealers into circles or squares of various sizes. A suggestive 

 remark in this connection is made by Dr S. Czapski when writ- 

 ing of the peculiar cover glass needed for use with Zeiss's latest 

 apochromatic objective of 1.63 N. A. He says, " The production 



