322 



NATURE 



\August 2, 1883 



specimen may be boiled for a short time. Longitudinal 

 sections may be cut by gluing the piece of stem to a cork 

 to afford a hold upon it. 



The preparation of sections of minerals and rocks is 

 usually considered a very difficult matter, but much may 

 be accomplished without the aid of the usual lapidaries' 

 wheel for cutting and the revolving lip for grinding the 

 sections, if the microscopist provides himself with a flat 

 piece of lead six to ten inches square, and two pieces of 

 boiler plate of the same size, planed on one side. A chip 

 of the rock may be ground flat on the leaden lap, charged 

 with coarse emery and water, and the process continued 

 with emery of moderate grain on one of the iron plates, 

 and the finest flour emery on the other. The flat side 

 being then cemented with balsam (undiluted) to a piece 

 of plate-glass about an inch square, the process of grinding 

 may be repeated on the other side of the chip, until it 

 becomes perfectly transparent. It may then be detached 

 from the glass by soa ing in benzole, and mounted in 

 balsam in the usual way. 



When the sections are to be mounted dry, which is 

 very rarely the case, the fine scratches left by the flour 

 emery must be removed by giving the section a final 

 polish on a hard and flat oilstone reserved for the purpose 

 and wetted with clean water only. 



When it is desired to preserve the natural colour of 

 objects, espe ially of such as contain chlorophyll, the 

 necessary preliminary treatment with alcohol raises an 

 objection to the balsam process, and another objection is 

 that some tissues are rendered too transparent, and many 

 of their finer features are obliterated by the highly refrac- 

 tive balsam. 



In these cases the object must be mounted in some 

 aqueous medium, the best and most convenient being a 

 preparation of glycerine and gelatine, which forms a trans- 

 parent jelly when cold, but is easily liquefied by heat. It 

 is best to buy this "glycerine jelly," as it is troublesome 

 to make on a small scale. When required for use it must 

 be liquefied by standing it in a cup of hot water. 



In general, objects to be mounted in glycerine jelly 

 should not be embedded, but if any support is needed in 

 cutting, this should take the form of two pieces of cork 

 hollowed out to the shape of the object. 



Water in the objects no longer presents any difficulty 

 in this method of mounting, but air has still to be con- 

 tended with, and the methods adopted for its elimination 

 in the balsam process are no longer applicable. Some 

 objects may be freed from air by boiling in water for a 

 few minutes, but many would be spoilt by such treatment. 

 Recourse must then be had to the air-pump, or, if this 

 instrument is not accessible, to a very simple process 

 depending on the great solubility of air in water. 1 



A wide-mouthed bottle of about four ounces capacity, 

 with a closely fitting solid stopper, is completely filled 

 with water, which at the time is, and for half an hour 

 previously has been, boiling, in order to expel all traces 

 of dissolved air. The stopper being then inserted with- 

 out inclosing a single air-bubble, the bottle is set aside 

 until cool enough to receive the sections, which are then 

 to be put into it. A few drops of boiling water are then 

 to be added to make good the inevitable loss in removing 

 the stopper ; the bottle is to be again closed, wiped dry, 

 and securely sealed with melted paraffin. After twelve 

 hours it may be opened, and the whole contents turned 

 into a white porcelain shallow dish. The sections can 

 then be easily seen, and picked out with a section-lifter, 

 and should be soaked for half an hour in a 50 per cent, 

 solution of glycerine before mounting. 



The process of transferring the object to the slide, 

 applying the liquefied jelly, and lowering the cover, are 

 exactly the same as in the balsam method, and the slide 

 should be set aside in a clip for a few hours for the jelly 



1 The writer cannot remember where he has seen this process described, 

 but he can testify to its efficiency. 



to solidify. In cold weather it is advisable to warm the 

 brass table by means of a spirit lamp, or the jelly may 

 viscify too quickly. 



When quite cold and set, the excess of jelly may be 

 cleaned from the edges of the cover glass, and the slide 

 may then be ringed with asphalte while running in the 

 turntable. Two or three subsequent coats of asphalte 

 and the attachment of labels will complete the slide. 



The objects for which gljcerine jelly is most suitable 

 are the lower forms of vegetable life — Algae, Desmidia- 

 ceae, Characeaa, Hepaticae, Fungi, Lichens, Mosses, &c, 

 and cuticles and sections of plants of all kinds. Many- 

 animal tissues are also better seen in it than in balsam. 



It is but seldom that other preservative media are re- 

 quired, and it will be found that almost all objects may 

 be suitably preserved by one of the three methods here 

 described. 



Closely related to the preparation of objects for micro- 

 scopic examination is their delineation by photography, 

 an art of the greatest value on account of its freedom 

 from bias and personal equation, and as a means of lecture 

 illustration with the aid of the lantern it must be appre- 

 ciated by the numbers who have experienced the diffi- 

 culty of demonstrating microscopic structure to many 

 persons. 



This application of photography, which is almost as 

 old as the photographic art itself, extending back to the 

 days of Daguerreotype, owes its recent development and 

 simplification mainly to the introduction of gelatine 

 plates, and the object of Mr. Malley's work 1 (which, how- 

 ever, should be called Photomicrography, for it does not 

 treat of the production of microscopic photographs, as 

 its name would imply) is to show how in an ordinary 

 room, with an ordinary microscope, photographic camera, 

 and paraffin lamp, photographs can be taken which will 

 bear comparison with those obtained in the old days by 

 the aid of sunlight reflected from expensive heliostats, 

 electric arcs, magnesium and lime light, microscopes of 

 special construction, and rooms specially set apart for the 

 work. It therefore appeals to a large class of persons — 

 those who would wish to practice the art, but lack either 

 the sunlight hours or the expensive illuminators and 

 apparatus formerly considered necessary. 



The microscope, camera, and dark room, with their 

 accessories, and the method of working with the Swan 

 incandescent lamp and sunlight are described in detail, 

 but the reader is perplexed by references to an illustra- 

 tion which cannot be found in the book. Four Wood- 

 burytype reproductions of photomicrographs of Aulauco- 

 discus, Pleurosigma, and Surirella, scales of Lepisma, 

 and Bacilli in human lung, accompany the work. 



The instructions for taking negatives by the wet col- 

 lodion and gelatino-bromide processes and the production 

 of positives, enlargements, &c, are clear and concise, but 

 we must enter an emphatic protest against the author's 

 opinion that in object-glasses for photomicrography, depth 

 of focus or penetration is to be sacrificed to angular aper- 

 ture. Penetration and flatness of field are really of greater 

 importance in lenses for photographic than for visual pur- 

 poses, for in viewing an object under the microscope the 

 observer has the power of focusing in rapid succession, 

 and by imperceptible gradations, points at different depths 

 and different distances from the centre of the field ; but a 

 photograph represents only such structures as were in 

 focus at the time of exposure, and once taken, the focus 

 is unalterable. It is therefore desirable to secure as great 

 a depth of focus and as flat a field as possible — qualities 

 which are incompatible with large apertures. 



Mr. Malley very properly advises his readers not to 

 walk about during the exposure of a plate, but the exten- 

 sion of the prohibition to speaking also is surely an 

 unnecessary restraint. 



1 " Micro-Photography," by A. Cowley Malley, B.A., M.B., &c. (Lewis. 

 Gower Street.) 



