rod, which, bent in a gentle curve, conducts the light from the flame to the under surface of the stage, the mirror in this case not being used. Perhaps the most useful lamp is that known as the albo-carbon light. It is the one I am in the habit of using when-especially in winter-good daylight is not available. Ordinary gas is used, but the gas has to traverse a chamber containing naphthaline before it reaches the burner. A very white light is thus obtained, and can be used without the intervention of blue glass or a blue copper solution. If expense be no bar, then a small incandescent electric light is most useful. 19. Dissecting Microscope (figs. 24, 25).-This is very useful. The lenses usually employed magnify from 5 to 20 times linear, and are fitted into a framework which can be raised or depressed, so as to bring the object distinctly into focus. 20. Method of Measuring the Thickness of Cover-Glasses.Thick cover-glasses are of no use, and thin ones-extra thin, so called-are to be pre clip projecting from the box fixes the cover-glasses, and the thickness is given by an indicator moving over a divided circle on the lid of the box. The divisions show hundredths of a millimetre. 21. Camera Obscura Shade.-When one has to continue observing a microscopic object for a long time, it is convenient to shade the eyes from all light reaching them, except that transmitted through the eye-piece. This is best done by a blackened screen either fixed to the microscope or arranged in front of the microscope.1 Some observers place the microscope in a dark chamber, allowing light to fall upon the mirror through an aperture in its front wall. Flögel 2 has designed a camera obscura for this purpose. 1 Schiefferdecker, Archiv f. wiss. Mikr., p. 180, 1892. III. NORMAL OR INDIFFERENT FLUIDS. Not unfrequently living tissues or fresh tissue elements have to be examined in as natural a condition as possible, and obviously, if a fluid require to be added to it, the fluid must be of such a nature that it will not injuriously affect the tissue or its elements. These fluids are spoken of as indifferent or normal fluids. When fresh, these fluids cause very slight changes in the tissues. Amongst those used are 1. Normal Saline.-Dissolve 6 grams of pure sodic chloride in 1000 cc. water. 2. Kronecker's Fluid.-It consists of Distilled water. 100 cc. .6 gram. .06 3. Blood Serum. -The blood is allowed to clot, and the serum, after a day or so, is poured away from the clot. In a laboratory provided with a "centrifugal apparatus" any red blood-corpuscles can be got rid of by "centrifugalising" the serum. This fluid does not keep long, and must be fresh. It has been suggested to add a piece of camphor to it, but this only helps to preserve it for a short time. Iodine is sometimes added to serum to form iodised serum, but this is by no means an indifferent fluid (p. 25). 4. Aqueous Humour. With a narrow triangular knife puncture the cornea of a freshly-excised ox's eyeball and collect the aqueous humour which exudes. If only a small quantity be required, it may be obtained by puncturing the excised eye of a frog with a fine capillary glass pipette (p. 4). 5. Fluid of Ripart and Petit :— Camphorated water Distilled water Glacial acetic acid It is very useful for examining animal cells. IV. DISSOCIATING FLUIDS. These fluids dissolve or soften the cement substance or interstitial material, e.g., of epithelium, connective tissue, and thus facilitate the separation of the histological elements from each other. In some cases, in isolating tissue elements, it is well not to have too much fluid in proportion to the morsel of tissue, else hardening is more apt to take place. 1. Strong Iodised Serum. A strong solution should be kept, which may be diluted as required. To the amniotic fluid of a cow or to blood-serum add crystals of iodine, and keep it in a stoppered bottle. Shake it frequently. At first very little iodine is dissolved, but after a time (15-20 days) the solution becomes much stronger, i.c., it becomes of a deep-brown tint. For dissociating a tissue, a weak iodised serum is used. A little of the strong fluid is added to fresh serum until the latter has a light-brown colour. If an object be placed in the dilute iodised serum and the brown colour fades, more of the strong solution must be added. In using this fluid, take a very small piece of tissue, the size of half a pea or less, and place it in 5 cc. of the fluid in a glass-stoppered bottle. After a day or two it may be dissociated with needles, but the brown tint must be maintained; more strong fluid must be added if putrefaction is to be prevented. 2. Dilute Alcohol (Ranvier's Alcool au tiers).—This fluid, devised by Ranvier, and sometimes called "one-third alcohol,” is of the greatest possible service, and is one of the best dissociating fluids we possess. Mix 1 part of 96 per cent. alcohol with 2 parts of distilled water. It dissociates epithelial and other tissues in 24-36 hours. Use a small quantity of the fluid in proportion to the tissue. 3. Chromic Acid.—One gram in 1000 of water. This requires two days to a week, according to the tissue placed in it. 4. Potassic Bichromate.-Two parts in 1000 of water, i.., .2 per cent. It is very useful for dissociating epithelium and the nerve-cells of the spinal cord. It does so in 2-3 days. 5. Ammonium Chromate (5 per cent.).—It is used for dissociating the "rodded" cells of the renal tubules, cells of salivary glands, Purkinje's fibres of the heart, &c. It acts in 24-36 hours, and the chromate must be well washed out of the tissues if they are to be preserved. 6. Caustic Potash.-Thirty to 35 parts in 100 of water. It acts in 20-30 minutes, rapidly destroying connective tissue. Water must not be added to the dissociated tissue, else the tissues are rapidly dissolved. It is used for isolating the fibres of smooth muscle, or heart-fibres. Examine the dissociated tissues in the dissociating fluid. As a rule, tissues so dissociated cannot be preserved, but there are certain exceptions. 7. Dilute Osmic Acid (.1 per cent.). It acts in 24-48 hours according to circumstances, and is well suited for cells containing fat globules. This is specially useful for the central nervous system. Small pieces must lie in it for 1-5 days. (See Lesson on Nervous System.) 9. Methyl Mixture or Schiefferdecker's Fluid : Methylic alcohol. Distilled water 5 cc. 50," 100 99 It is better to prepare it fresh, but it can be preserved in a glass-stoppered bottle. The methylic alcohol rapidly evaporates. The tissues remain in it for several days, and it is specially useful for the retina and central nervous system. I find that it isolates epithelial cells in one to two days. It acts very much like Ranvier's alcohol. 10. Other fluids are referred to in the text, e.g., baryta water, and 10 per cent. sodic chloride for tendon. This dissolves the cement substance of epithelial cells and the mucinoid cement of connective tissue. 11. Digestion Methods, both gastric and tryptic, FIG. 27.-Glass are used (see text). Tube or The special uses of the above-named fluids are dissociating referred to in text under the tissues or organs, for small pieces which each is specially adapted. of a tissue. General Directions for Dissociating Tissues.Always use a very small piece of the tissue or organ, and place it in a not too large quantity of the dissociating fluid. thimbles (fig. 27) are very useful for this purpose. Small glass V. HOW TO TEASE A TISSUE. To separate by means of needles the elementary parts of a tissue is by no means an easy task. The tissue must be seen distinctly, and the needles must be so used as not to break up the parts, but only to separate them. The process may be done by the unaided eye or with the aid of a lens or dissecting microscope (figs. 24, 25). The light must be good, and an appropriate background for the object should be selected. If the tissue is colourless, use a black In the background, e.g., black paper; if coloured, a white one. latter case, however, the shadows interfere with exact vision, and it is better to support the slide upon an object raised slightly above the white background. This is readily accomplished by placing it over a white porcelain capsule, or on a photophore (fig. 28), into which is slipped a piece of white paper. In a laboratory, one of the most convenient ways is to have the tables painted of a black or very dark green colour, but the painting must be flat, with no shining varnish. At the edge of the table is painted a white strip 1 inches broad. Some prefer to burn into the surface of the table solid paraffin blackened by means of lampblack. Others use porcelain slabs one-half black and the other white. Photophore (fig. 28).—This is a small wooden box, 5 cm. high, 9 cm. long, and 9 cm. broad. The upper part is formed of glass. The front wall of the box is wanting. Placed obliquely within the box is a mirror, which reflects the light upwards through the glass cover and the slide to of the observer. the eye In dissociating with needles, we must have some knowledge of the arrange FIG. 28.-Photophore. ment of the parts of the object to be teased, such as the direction of the fibres, &c. Take a small piece only. Always tease one end of the tissue, and fix the latter with one needle while the parts are separated with the other needle. One of the most convenient combinations is that of Eternod, which combines a photophore with a turntable. The wheel of the latter can be removed, and the block forms not only a photophore, but also a surface on which tissues of different colours can be teased and mounted. VI. FIXING AND HARDENING REAGENTS. Most of the tissues and organs must be hardened in suitable fluids before they can be cut into sections. A large number of fluids of various kinds are used, each organ or tissue requiring its own appropriate fluid. Some organs, e.g., bone, are too hard to be cut in their natural condition; they must be decalcified by appropriate fluids. Amongst others, the following fluids are required, but others are referred to in the text. A. Alcohol. Alcohol is one of the most important hardening fluids used |