Sometimes slides are used which are made by taking a thin slip of wood of the usual size (3 in. by 1 in.), in the centre of which is cut a circular hole large enough to receive the object. A piece of thin glass is fixed underneath the slide, forming a cell for the object, which may then be covered and finished like an ordinary slide. This has the advantage of serving for transparent objects for which the before-mentioned wooden slides are unsuitable. A slight modification of this plan is often used where the thickness of the objects is inconsiderable, especially with some of the Diatomaceæ, 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 the 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 the 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 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 interfered with.