INJECTION. 122 CHAPTER VII. MISCELLANEOUS. 140 INDEX. 153 THE PREPARATION AND MOUNTING OF MICROSCOPIC OBJECTS. CHAPTER I. APPARATUS. Before entering into the subject of the setting of Objects for the Microscope, the student must be convinced of the necessity of cleanliness in everything relating to the use of that instrument. In no branch is this more apparent than in the _preparation_ of objects; because a slide which would be considered perfectly clean when viewed in the ordinary way is seen to be far otherwise when magnified some hundreds of diameters; and those constant enemies, the floating particles of dust, are everywhere present, and it is only by unpleasant experience that we fully learn what _cleanliness is_. Any object which is to be viewed under the microscope must, of course, be supported in some way--this is now usually done by placing it upon a glass slide, which on account of the transparency has a great advantage over other substances. These “slides” are almost always made of one size, viz., three inches long by one broad, generally having the edges ground so as to remove all danger of scratching or cutting any object with which they may come in contact. The glass must be very good, else the surface will always present the appearance of uncleanliness and dust. This dusty look is very common amongst the cheaper kinds of slides, because they are usually “sheet” glass; but is seldom found in those of the quality known amongst dealers by the name of “patent plate.” This latter is more expensive at first, but in the end there is little difference in the cost, as so many of the cheaper slides cannot be used for delicate work if the mounted object is to be seen in perfection. These slides vary considerably in thickness; care should, therefore, be taken to sort them, so that the more delicate objects with which the higher powers are to be used maybe mounted upon the _thinnest_, as the light employed in the illumination is then less interfered with. To aid the microscopist in this work, a metal circle may be procured, having a number of different sized openings on the outer edge, by which the glass slides can be measured. These openings are numbered, and the slides may be separated according to these numbers; so that when mounting any object there will be no need of a long search for that glass which is best suited to it.