362 The Microscope. 



most apt to give rise to mistakes is in the stud^^ of bacteriologi- 

 cal forms. One fully conversant witli the varieties and structure 

 of bacteria, would not be apt, it is true, to confuse these imper- 

 fections in their own eyes with what might be actually under the 

 microscope, but I not only know of this mistake having been 

 made by those who are not without experience in bacteriological 

 work, but it is not impossible for the same to occur to many 

 whose observation has not been sharpened by long practice in 

 the use of the microscope. I feel confident that, out of a dozen 

 persons who make the drawings of what thc}^ see of such numer- 

 ous, ill-defined objects, there would be two or three who would 

 figure what is not in the field of vision, but ah imperfection 

 which actually exists in their own eyes. 



ELEMENTARY MICROSCOPICAL MOUNTING.— IL 



DR. A. M. WEBSTER. 

 THE THIN GLASS COVER. 



rpHE thin glass is used, except in some very rare cases, as a 

 -A- cover to all mounts of microscopical objects. These rare 

 instances will be referred to hereafter. The purpose of the 

 thin cover is at least fourfold. It is really a cover to protect 

 the object beneath from dust and from accident, and it may be 

 used and often is used as a means of marking the position of 

 the minute object beneath it, so that another microscopist, to 

 whom the slide may be sent, can easily and rapidl}^ find that 

 object and see what his friend at a distance has seen. This is 

 done by finding the object, or a special part of it, with a low 

 power, and then placing on each side, but on the upper surface 

 of the cover glass, a minute ink spot. The friend to whom it 

 may then be sent will need only to put the preparation under 

 his microscope and find the ink spots, when he will have the 

 object in or near the position desired by him that sent it. But 

 this is not the chief purpose of the cover. It is only one of 

 those things that have suggested themselves after the cover had 

 been applied with other aims and for other reasons. 



As a rule objects are mounted in liquid preservative media, 

 or in substances that are fiuid when first used. There are 



