The Microscope. 333 



It is only within comparatively recent years that microscop- 

 ical objects have been monnted between glass. In the not 

 remote past the}^ were placed between thin disks of mica, that 

 in turn were fastened within circular apertures in a strip of 

 bone or of ivory. Such mounts are sometimes seen in the pos- 

 session of those that still have the old-fashioned solar micro- 

 scope, that was once used in class rooms and by lecturers. 

 Then came an improvement made by the use of glass somewhat 

 as we now use it, but not with the refinement and the delicacy 

 that the modern microscopist can bring to bear upon his prepa- 

 rations, for while the lower glass slip resembled that now used, 

 the upper one was as thick as ordinary window glass; it was, in 

 reality, a part of the same material from- which the lower slip 

 had been cut. Such objects were necessarily restricted to exam- 

 inations with very low powers of the microscope, the high power 

 objectives still demanding the use of a film of mica as the cover 

 to the object. This arrangement survived until there was dis- 

 covered in an English factory a method of making glass so 

 exceedingly thin that more than two hundred layei's of it must 

 be piled upon one another to make them measure an inch in 

 height. A single piece of this thin glass may therefore be only 

 the one two-hundredth of an inch or less in thickness. This is 

 excessively thin and not easily handled, as the reader may 

 imagine. It is very brittle, yet it is needed to cover certain 

 objects that must be examined with the verj^ high powers now 

 demanded by the advanced microscopist. The object is placed 

 on the lower slip, which is usually about the twentieth of an 

 inch in thickness, and the thin glass film is placed above it like 

 a cover, and in the same way as was formerly done with the 

 piece of window glass that was used for the upper layer, or the 

 cover, as it is commonly called. The object, the lower glass 

 slip and the cover glass constitute the slide, or, as the micro- 

 scopist sometimes calls it, the " mount." Each of these parts 

 claims careful attention. 



THE SLIP. 



The lower strip of glass — the one upon which the object is 

 usually placed — is the slip ; only after the object has been 

 permanently mounted and the preparation is finished, does it 

 become the slide. The slip is usuall}^ three inches long by one 

 inch wide, this being the standard size, and in common use in 



