16 PREPARATION AND MOUNTING 



are separate or of different kinds ; those of cardboard and wood 

 are generally free from this fault, yet the slides, being opaque, pre- 

 vent the employment of the Lieberkuhn. To some extent glass 

 slides, when covered with ornamental papers, are liable to the same 

 objection, as the light is partly hindered. And spmetimes the 

 dampness from the paste, or other substance used to affix the pa- 

 pers, penetrates to the object, and so spoils it, though this may be 

 rendered less frequent by first attaching the thin glass to the slide 

 by some harder cement. Much time, however, is taken up by the 

 labour of covering the slides, which is a matter of consideration 

 with some. Certainly, the cost of the glass slides was formerly 

 great ; but now they are reasonable enough in this respect, so that 

 this objection is removed. It is, therefore, well to use glass slides, 

 except where the thin glasses are employed for tests, &c., as above. 

 When the thin glass circles are placed upon the slides, and the 

 edge is varnished with black or coloured rings, the appearance of 

 finish is perfect. The trouble is much less than with most of the 

 other methods, and the illumination of the object very slightly in- 

 terfered with. 



To varnish the edges of these covers, make circles of any 

 liquid upon the glass slide, and perform any other " circular " 

 work mentioned hereafter, the little instrument known as " Shad- 

 bolt's turntable " is almost indispensable. It is made as follows : 

 At one end of a small piece of hard wood is fixed an iron 

 pivot about one-eighth inch thick, projecting half an inch from 

 the wood, which serves as a centre upon which a round brass 

 table three inches in diameter revolves. On the surface of this 

 are two springs, about one and a half inches apart, under which 

 the slide is forced and so kept in position, whilst the central part 

 is left open to be worked upon. JThe centre is marked, and two 

 circles half an inch and one inch in diameter are usually deeply 

 engraved upon the table to serve as guides in placing the slide, 

 that the ring may be drawn in the right position. When the 

 slide is placed upon the " table" underneath the springs, a camel- 

 hair pencil is filled with the varnish, or other medium used, and 

 applied to the surface of the glass ; the table is then made to 

 revolve, and a circle is consequently produced, the diameter of 

 which it is easy to regulate. 



