xxiv INTRODUCTION. 



loss of time. The cabinet should be furnished with two folding doors, so as to exclude 

 dust as much as possible. It should also be made of thoroughly seasoned wood, oak or 

 mahogany being the best ; if made of deal or cedar, the vapour of the volatile oil of the 

 wood will insinuate itself beneath the thin glass cover and the slide in those objects which 

 are mounted in the dry state, and, condensing upon them and the objects, will obscure and 

 spoil them. 



It may be remarked here that the names of objects should always be written upon labels 

 pasted (not gummed) to the slides, not merely upon the slides with a diamond. The 

 colour of the labels should be different for each kind of object; or if the labels be com- 

 posed of white paper, they should have a coloured margin ; thus those of the Desmidiacere 

 may be green, the Piatomacese yellow, c., so that the various slides, when acciden- 

 tally mixed after comparative examinations, can be readily replaced in their respective 

 drawers. 



Bell-ylasses. The microscope when in use, either constant or occasional, should always 

 be kept under a large bell-glass, the base of which fits into an annular groove made in a 

 circular flat wooden stand. In this way it is kept from dust, and the trouble and wear 

 and tear consequent upon putting it into a box is saved. Moreover, thus protected, an 

 object under examination can be left without fear of injury or disturbance, and be also 

 preserved from dust. 



Several smaller bell-glasses of various sizes should also be kept at hand, under which any 

 objects which it may not be convenient to mount for a time, or the examination of which may 

 not be completed, can be protected. 



Slides. These are ordinarily made of glass about the thickness of common window-glass j 

 their length is usually 3 inches, and their breadth 1 inch. The old length was "1\ inches, 

 which I prefer, as the longer slides, with a large number of objects, take up so much room ; 

 but as the aperture in the stage has been enlarged in the modern microscopes to allow of 

 the passage of the parabolic reflector, the Amici's prism, &c., if the old size be retained 

 the slides will drop through the stage : this we remedy by an additional brass plate. 

 Where the objects are very large, the slide must be proportionately large, and its thickness 

 greater than usual. The slides should be made of colourless glass, so as not to interfere 

 with the appreciation of the colour of an object. And they should be flat j otherwise the 

 parts of the object will lie in different planes, and every motion of the slide will require 

 new adjustment of the focus. The edges are best somewhat ground on a copper plate with 

 emery, to prevent injury to the fingers or scratching the stage-plate. Very delicate 

 structures require to be examined and mounted upon thin glass. The slides may then be 

 made of wood, sheet zinc, or tin-plate, with a circular aperture in the middle, upon 

 which a piece of thin glass is cemented ; or slides of 9~10-oz. crown-glass may be used. 



Covers. Comparatively few objects can be viewed in the dried state ; hence they are 

 most frequently immersed in some kind of liquid. To prevent the evaporation and conden- 

 sation of this upon the object-glass, and to reduce the thickness of the layer of liquid to a 

 minimum, the obj ect is usually covered with a piece of thin glass. The form of this cover is 

 either square or circular, and the thickness from about the :5 V to the ?^$ f an mc fy OP 

 even less. These covers are usually kept already cut by the microscope-makers and those 

 who sell objects. Before use, they are best allowed to remain immersed in water for some 

 time. Care is required in wiping this thin glass. It is usually effected by holding the 

 cover at two opposite points of the margin between the finger and the thumb of the left 



