~Microscopical Slides.~--The slides in general use are those known as "three by one" slips (measuring 3 inches by 1 inch, or 76 by 26 mm.), and should be of good white crown glass, with ground edges. ~New slides~ should be allowed to remain in alcohol acidulated with 5 per cent. hydrochloric acid for some hours, rinsed in running water, roughly drained on a towel, dried, and finally polished with a selvyt cloth. [Illustration: FIG. 23.--Slides and cover-slips, actual size.] If only a few slides are required for immediate use a good plan is to rub the surface with jeweler's emery paper (Hubert's 00). A piece of hard wood 76×26×26 mm. with a piece of this emery paper gummed tightly around it is an exceedingly useful article on the microscope bench. ~Cover-slips.~--The most useful sizes are the 19 mm. squares for ordinary cover-glass film preparations, and 38 by 19 mm. rectangles for blood films and serial sections; both varieties must be of "No. 1" thickness, which varies between 0.15 and 0.22 mm., that they may be available for use with the high-power immersion lenses. Cover-slips should be cleaned in the following manner: 1. Drop the cover-slips one by one into an enamelled iron pot or tall glass beaker, containing a 10 per cent. solution of chromic acid. 2. Heat over a Bunsen flame and allow the acid to boil gently for twenty minutes. NOTE.--A few pieces of pipe-clay or pumice may be placed in the beaker to prevent the "spurting" of the chromic acid. 3. Turn the cover-slips out into a flat glass dish and wash in running water under the tap until all trace of yellow colour has disappeared. During the washing keep the cover-slips in motion by imparting a rotatory movement to the dish. 4. Wash in distilled water in a similar manner. 5. Wash in rectified spirit. 6. Transfer the cover-slips, by means of a pair of clean forceps, previously heated in the Bunsen flame to destroy any trace of grease, to a small beaker of absolute alcohol. Drain off the alcohol and transfer the cover-slips, by means of the forceps, to a wide-mouthed glass pot, containing absolute alcohol, in which they are to be stored, and stopper tightly. NOTE.--After once being placed in the chromic acid, the cover-slips must on no account be touched by the fingers. ~Used Slides and Cover-slips.~--Used slides with the mounted cover-slip preparations, and cover-slips used for hanging-drop mounts, should, when discarded, be thrown into a pot containing a 2 per cent. solution of lysol. After immersion therein for a week or so, even the cover-slips mounted with Canada balsam can be readily detached from their slides. _Slides._-- 1. Wash the slides thoroughly in running water. 2. Boil the slides in water to which "sapon" has been added, for half an hour. 3. Rinse thoroughly in cold water. 4. Dry and polish with a dry cloth. _Cover-slips._-- 1. Wash the cover-slips thoroughly in running water.