ment the amount of water supplied to each specimen can be easily regulated.

Slides should be of colorless glass with ground edges. The English form, measuring IX 3 inches (76 × 26 mm.), is to be preferred for ordinary use. Occasionally broader slides are needed. Thick slides are preferable to thin ones; the latter are so light that they are easily lifted by the oilimmersion lens; they also seem to warp when heated to attach paraffin sections to them.

Cover-slips should be square or oblong according to the shape of the specimen. Most dry lenses are adjusted for cover-glasses measuring 16 or 17μ in thickness, so that if possible no cover-slips ranging outside of 15 to 18μ should be used. With an oil-immersion the exact thickness is not quite so important.

Slides and cover-slips are cleaned by dipping in alcohol and wiping dry with a soft crash towel or old linen handkerchief.

To clean old slide preparations, heat them until the balsam softens so that cover-slips and slides can be drawn apart. The slides and cover-slips are then treated separately with nitric acid. A 10 per cent. solution is usually sufficient, but occasionally the strong acid will be found necessary. Some workers prefer equal parts of alcohol and hydrochloric acid; still others the following mixture:

A thorough washing in running water, followed by alcohol, completes the process. Alkalies are not so good for cleaning purposes, because they attack the glass.

Staining Dishes.-Watch-glasses are not satisfactory on account of their instability. Concave dishes with flat bottoms are much better for ordinary use, and can be obtained of several patterns. They should be large enough. to hold 25 c.c. of fluid. The Syracuse solid watch-glasses are very good dishes of this shape.

Stender dishes of various sizes will be found useful for many purposes.

The "New Practical Staining Dish" (No. 16,618) manufactured by the Bausch and Lomb Optical Co. is very useful for staining at once a number of paraffin sections.

Oblong rectangular Petri dishes are very convenient for staining preparations mounted on the slide.

For staining at once a large number of sections for class purposes, the Hobb's Tea Infuser has been found very useful. If set in a small lemonade cup, but little stain is required.

Large concave dishes holding 1000 c.c. will be found the most convenient for holding frozen sections of fresh tissue, because a slide can be dipped into them and under the sections.

Metal Instruments.-Spatulas of different sizes are

FIG. 108.-Spatula.

needed. They should be thin, smooth, and large enough, so that a section will not curl over the edge (Fig. 108).

The best instrument for transferring sections under all cir

cumstances is a piece of platinum wire mounted in an ordinary screw needle-holder. It is pliable and can be bent to any shape, will not break like a glass needle when dropped, and is not affected by acids. but serviceable substitute. desired shape by heating. many other instruments required in microscopical work do not need any special mention.

Ladies' hat-pins form a cheap They are readily bent to any Forceps, scissors, scalpels, and

Bottles. For cover-slip work and for staining on the slide dropping-bottles will be found extremely convenient. The patent T. K. pattern of 50 c.c. capacity, with flat top, is probably the best form and size.

EXAMINATION OF FRESH MATERIAL.

Fresh tissues may be examined either in teased prepara

tions or in sections.

Teased preparations are made by cutting out a very small bit of the tissue in question and dividing it as finely as possible, by means of two sharp, clean needles, on a slide in a drop or two of some indifferent fluid, such as the normal salt solution. Teased preparations are often made, for instance, of the heart-muscle when fatty degeneration is suspected. If the tissue is soft, the cells are easily obtained by simply scraping the cut surface with the edge of the knife.

Sections of fresh tissues can be made with a razor or with a double knife, but much the better way, at least for general diagnostic purposes, is to use frozen sections, which can be very quickly and perfectly made with the freezing microtome. The fresh sections are put into salt solution in a glass dish large enough to permit of a slide being dipped into it, so that a section can be floated and spread out evenly on its surface. The slide is then carefully raised, the excess of fluid wiped off, and a cover-slip put on.

If it is desired to stain the section, a few drops of Löffler's methylene-blue solution are poured over it after it is spread

evenly on the slide. In a few seconds the coloring fluid is thoroughly washed off with salt solution, a cover-slip put on, and the section examined in the salt solution. If sections of fresh tissues are put directly into a staining fluid in the ordinary manner, they pucker up and do not stain evenly.

Sections of fresh tissue may be fixed, stained, cleared, and mounted in balsam by a slight modification of the method for frozen sections given on p. 256. This modification consists in covering the section with 95 per cent. or absolute alcohol after it has been spread out evenly on the slide as described in the method referred to. The alcohol is to be dropped on the section carefully from a drop bottle, in order to avoid folding. After thirty seconds the alcohol is drained off and the section flattened out on the slide with blottingpaper and further treated according to the method above mentioned. If the section is not treated with alcohol before blotting, it will adhere to the blotting-paper and not to the slide.

Fresh preparations are often treated with chemicals for various purposes. Of these chemicals, acetic acid is the most generally useful in pathological work. It shrinks the nuclei and renders their outlines more distinct. It swells connective tissue, making it more transparent, so that the elastic fibers which are unaffected stand out distinctly. It precipitates mucin and dissolves or renders invisible the albuminous granules so abundantly present in the protoplasm in the cloudy swelling of various organs in disease. Its main use as a reagent for fresh tissues is to demonstrate fat and to differentiate that substance from albuminous granules.

Acetic acid is ordinarily used in a 1 to 2 per cent. aqueous solution, a few drops of which are placed at one edge of the cover-slip, and then drawn beneath it by placing a piece of filter-paper on the opposite side. If in a hurry, however, stronger solutions, or even glacial acetic acid, may be used. Other reagents are of less importance, but are occasionally used.

Osmic acid is sometimes employed in a 1 per cent. aque

ous solution to demonstrate fat, which it stains brown to black.

An alcoholic solution of Scharlach R. or Sudan III. is being used more and more for the same purpose.

fat orange to red.

It stains

Hydrochloric acid in a 3 to 5 per cent. solution is used to demonstrate calcification. Phosphate of lime is simply dissolved, while from carbonate of lime bubbles of carbondioxid (CO2) are set free.

Indifferent Fluids.-Fresh tissues are usually examined in normal salt solution-a per cent. solution of common salt in water. It has the advantage over water that tissues do not swell up so much in it, blood-globules are unaffected, and the finer structures are better preserved. A very few drops of Lugol's solution added to the stock-bottle of salt solution will be found useful in preventing the growth of mould.

Serous fluids, such as hydrocele fluid, are occasionally used. Artificial serum is made by adding 1 part of eggalbumin to 9 parts of normal salt solution.

Macerating fluids are little used in pathology. Occasionally, however, when tissues are tough, so that they cannot be readily teased apart, they are macerated in certain fluids which dissolve the substances that hold the different elements together. The reagents most commonly used are the following:

1. Ranvier's one-third alcohol is made by taking I part of 96 per cent. alcohol and 2 parts of water; twenty-four hours are usually enough.

2. Very dilute solutions of chromic acid are recommended -1 to 3 of I per cent.

3. 33 per cent. Caustic Potash.-Tissues are macerated in a few minutes to one hour: they must be examined in the same fluid, because the cells are destroyed if the solution is weakened.

Examination of Fluids.-Small fragments of tissue should be picked out with forceps. If much blood is adherent, wash the tissue well in salt solution. When the cell