SLIDES, COVERS, ETC. xxi 



pasted to the slides, not merely upon the slides with a diamond ; and that the colour of the 

 labels should be different for each kind of object ; or if the labels be composed of white paper, 

 they should be impressed by a coloured margin ; thus those of the Desmidiacese green, the 

 Diatomacese yellow, &c., so that the various slides when accidentally mixed, after compara- 

 tive examinations, can be readily replaced in their respective drawers. 



Bell-glasses. 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, as thus protected, an 

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

 served 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 ; 

 then- length should be 2 inches, and their breadth 1 inch. If longer than this, the ends 

 are liable to project beyond the sides of the stage, and may be displaced accidentally by 

 passing the hand to the mirror, or condenser, and they occupy more space in a cabinet than 

 when of the above size ; moreover, there is no possible advantage to counterbalance these 

 disadvantages. 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, 

 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 ground somewhat, to prevent 

 injury to the fingers. Very delicate structures require to be examined and mounted upon 

 thin glass. The slides may then be frequently made of wood, sheet-zinc, tin-plate, or card- 

 board, with a circular aperture in the middle, upon which a piece of thin glass is cemented. 



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

 frequently immersed in some kind of liquid. To prevent the evaporation and condensation 

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

 the object 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 -^ih to the ^ n dth of an inch, 

 or 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 

 right hand, and rubbing the surfaces with a fold of a cloth, leather, or silk handkerchief 

 covering the same parts of the left hand. But the thinnest glass cannot be wiped in this 

 way without being broken. This requires to be held at the edge by the finger and thumb 

 of the right hand applied to the flat surfaces, and to be drawn slowly through the fold of the 

 cloth in the left hand as above. 



Dipping-tubes. These are glass tubes varying in length from about 5 inches to a foot, 

 and in calibre from ^ to i an inch. They are cut of the proper length by a three-square file, 

 and the ends gently fused in the flame of a spirit-lamp. One end is then coated outside 

 with sealing-wax and spirit, or some other coloured liquid, so that the same end may always 

 be used for the same purpose. They are of use for removing objects from water or other 

 liquids in which they may be contained. Suppose, for instance, it is required to examine 

 some deposit lying at the bottom of a liquid, or an object suspended, the fore-finger of the 

 hand in which the tube is held is placed upon the upper end of the tube so as to close it ; 



