32 PREPARATION AND MOUNTING 



of the objects is inconsiderable, especially with some of the 

 Diatomacese, often termed " test-objects." The wooden 

 slide is cut with the central opening as above, and two 

 pieces of thin glass are laid upon it, betwixt which the 

 diatoms or other objects are placed, and kept in their proper 

 position by a paper cover. This arrangement is a good 

 one, insomuch as the very small portion of glass through 

 which the light passes on its way to the microscope from 

 the reflector causes tho refraction or interference to be 

 reduced to the lowest point. 



A novice would naturally think the appearance of some 

 of the slides above mentioned very slovenly and unfinished ; 

 but they are often covered with ornamental papers, which 

 may be procured at almost every optician's, at a cost 

 little more than nominal, and of innumerable patterns and 

 colours. How to use these will be described in another 

 place. 



It is very probable that a beginner would ask his friend 

 what kind of slides he would advise him to use. Almost 

 all those made of wood are liable to warp more or less, even 

 when the two pieces are separate or of different kinds; 

 those of cardboard and wood are generally free from this 

 fault, yet the slides, being opaque, prevent 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 sometimes dampness 

 from the paste, or other substance used to affix the papers, 

 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 



