AugUSt 2, 1883] 



NATURE 



321 



cloud, and noticed that it had what I may couipare to a bow- 

 string stretched from end to end. On Thursday, July 19, from 

 II to 12 p.m., the whole sky was divided by such band; con- 

 verging east and west. This was noticed by many persons in 

 Essex, where I was staying, E. C. Wallis 



31, Meadow Road, S.W. 



ON MOUNTING AND PHOTOGRAPHING 

 MICROSCOPIC OBJECTS 1 



II. 



THE prepared slide fixed in a clip should be placed on 

 a hob or in a cool oven (not above 50 C.) for two 

 days, by which time the excess of balsam round the edge 

 of the cover will have become brittle, and can be removed 

 with the point of a scalpel or penknife. Any balsam still 

 remaining can be cleaned off with methylated spirit and 

 a clean soft rag. The final cleaning of the slide may be 

 done with soap and water. As the balsam itself serves 

 to secure the cover to the slide, no cement or varnish is 

 needed, and it remains only to label the object. 



After successfully mounting this object, no difficulty 

 will be experienced in applying the same methods to 

 other small insects and parts of insects, such as antenna;, 

 spiracles, feet, wings, ovipositors, corneas, tracheal, &c. 

 The two last cases, however, require careful dissection. 



Animal hairs are best mounted in balsam, and the only 

 special treatment they require is soaking for a short time 

 in ether to remove grease. 



The siliceous skeletons of diatoms and spiculae of 

 sponges and Holothuriae require cleaning from extraneous 

 matter by treatment with strong acids, but space will not 

 allow a description of the details of their preparation. 



The mounting of the organs and tissues of the higher 

 animals and plants should not be attempted until tolerable 

 facility has been acquired in the preparation of the 

 simpler objects previously mentioned, as their structure 

 is usually revealed only by the somewhat difficult process 

 of cutting thin sections of them. 



Most animal substances require hardening before they 

 can be cut. Hardening may be thus effected. The per- 

 fectly fresh tissue is to be cut into pieces about the size 

 of Spanish nuts, and soaked in ten times its bulk of a 

 solution, consisting of one part of methylated spirit, and 

 two parts of a A per cent, solution of chromic acid. At 

 the end of twenty-four hours, and again after every third 

 day, the solution is to be changed. After a week or 

 fortnight the pieces should be well washed in methylated 

 spirit, and will then be hard enough for cutting. 



The next process is to embed the tissue in some sub- 

 stance firm enough to afford it support, yet soft enough 

 to be readily cut with it. A good material for this purpose 

 is a mixture of three or four parts of solid paraffin (paraffin 

 candles), three of lard, and one of paraffin oil. It should 

 be heated just sufficiently to keep it fluid, and the hardened 

 tissue from which the exc-ss of alcohol has been drained 

 should be soaked in it for a quarter of an hour if of 

 moderately close texture. If of very open texture— lung 

 or testis, for instance — it must be soaked for about half an 

 hour in rectified alcohol, and for a like period in absolute 

 alcohol, to remove all traces of water. Then after dis- 

 placing the alcohol by a quarter or half an hour's immer- 

 sion in oil of turpentine, the tissue may be placed in the 

 melted wax, which being readily miscible with the 

 turpentine, will gain access to all the interstices of the 

 substance. 



A mould must then be prepared by gumming a piece of 

 paper round a cork or cylinder of wood, the paper being 

 allowed to project about three-quarters of an inch. Into 

 this mould the substance is to be put, and the space filled 

 up with some of the melted wax. When quite cold the 

 paper may be stripped off, and the preparation will be 



1 Concluded from p. 303. 



ready for cutting with a razor, wetted with spirit to prevent 

 adhesion of the sections. 



The sections as they are cut are to be floated off the 

 razor into methylated spirit, from which they may be 

 transferred to a staining fluid. 



The object of staining is in most cases not simply to 

 impart a general colour to the object, but to take ad- 

 vantage of the fact that different parts are affected in 

 different degrees by the same dye and are thereby clearly 

 discriminated. Thus if an ammoniacal solution of car- 

 mine be employed, the structures which are first and most 

 deeply stained are nuclei, axis cylinders of nerves, and 

 ganglion corpuscles. To a less extent it stains the proto- 

 plasm of gland-cells and connective tissue corpuscles. 

 But if the action be too long continued, the whole will be 

 deeply and uniformly stained, and the selective power will 

 be lost. 



Carmine solution may be prepared by dissolving with 

 the aid of gentle heat 2 grammes of carmine in 4 c.c. of 

 ammonia and 48 c.c. of distilled water. Continue the heat 

 or expose to the air until the smell of ammonia has 

 almost disappeared, and then keep in a well-corked bottle. 

 When required for use, a few drops of this solution should 

 be added to a watch-glass full of water. 



Logwood resembles carmine in its action and is by 

 many preferred to it. It may be prepared as follows : — 

 12 grammes of extract of logwood and 36 grammes of 

 alum, both in fine powder, are to be mixed with 60 c.c. of 

 distilled water, stirred well with a glass rod and filtered. 

 Add to filtrate 5 c.c. rectified alcohol. Dilute with two 

 or three times its volume of distilled water when used. 

 When the tissue has been hardened with chromic acid, 

 the sections should be soaked for a few minutes in a I 

 per cent, solution of sodic bicarbonate to neutralise the 

 acid before staining in logwood. 



No general rule can be given for the length of time the 

 section must remain in the staining fluid. It will vary 

 from a few minutes to as many hours, and the section 

 must be removed and examined with the microscope from 

 tine to time to see when the process has gone far 

 enough. 



When sufficiently stained, the excess of staining fluid 

 is to be drained off and the section passed through recti- 

 fied spirit 60 O. P., oil of cloves, and oil of turpentine, 

 remaining about five minutes in each, and may then be 

 mounted in balsam as already described. 



For displaying tesselated epithelium in mesenteries, 

 lungs, and blood-vessels, nothing can be more beautiful 

 than staining by oxide of silver reduced from the nitrate. 

 The perfectly fresh membrane or the section of hardened 

 tissue as the case may be must be well washed with dis- 

 tilled water and then soaked for five minutes in a 5 per 

 cent, solution of nitrate of silver. It is then again to be 

 washed and exposed in distilled water to sunlight until it 

 assumes a brown colour. The necessary exposure will 

 vary from ten minutes to an hour or more. After a final 

 wash in distilled water, it may be treated like objects 

 stained by other methods. By this treatment the tissue 

 assumes a general pale brown tint and the outline of 

 every cell is sharply marked out by a deep brown 

 deposit of argentic oxide in the intercellular substance. 



Many vegetable tissues, such as cork, pith, succulent 

 leaves, and some fruits, tubers, and roots, can be cut 

 without previous preparation, and for such as are too soft 

 to be cut in the fresh state the process of hardening is 

 simpler than that employed for animal substances. De- 

 hydration by simply soaking for a day or two in methy- 

 lated spirit usually suffices. 



Stems of plants usually require softening before cutting, 

 and this softening can be effected if the wood is young 

 by two or three days' immersion in methylated spirit to 

 remove resinous matter, followed by maceration for from 

 four days to a week in water. When the wood is old or 

 unusually hard, the maceration must be prolonged or the 



