Feb. 1, 1SGG.J 



SCIENCE-GOSSIP. 



33 



the object as depicted upon the field-glass of the 

 eyepiece in total fog. The best object-glasses for 

 use with these instruments appear to me, to be the 



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 4) ~oi <T> 8) " I11 - 1 ig" 



This brief notice of illumination for opaque 

 objects with high powers must not close without 

 reference to the last method previously mentioned, 

 viz., the reflector and illuminator, which appears to 

 possess certain elements extremely valuable. The 

 glare is much reduced, and necessarily so, as the 

 image of the object does not pass through any 

 plate of glass, the binocular prism simply directing 

 the rays of light into the objective, and thence to 

 the object ; the magnified figure being seen through 

 the right-hand tube without any medium being 

 interposed. There are one or two drawbacks to 

 this plan. Eirst, nothing can be seen biuocularly, 

 although it is well known that a ^ and ^ may be 

 used with advantage in some cases in the binocular ; 

 of course, as one tube is taken up with the illumi- 

 nating apparatus, the observer can only make use 

 of the other. Secondly, as the prism cuts off half 

 the pencils of light refracted through the object- 

 glass, its power is sensibly diminished. Thirdly, it 

 is somewhat difficult to get a field equably illumi- 

 nated. The arrangement is nevertheless very 

 simple and efficacious where the binocular is not 

 required. Owuers of these contrivances must not 

 rashly condemn either, as a great deal depends, as 

 before mentioned, upon manipulation ; and there 

 can be no doubt but that they place in our hands as 

 microscopists, a new power, to be hereafter used in 

 the solution of certain points which, without their 

 aid, would remain in ambiguity. 



John Bockett. 



MOUNTING CRYSTALS. 



BY A. J. BOBERTS. 



rpiIE mounting of crystals, though at first 

 -*- sight apparently simple, is frequently attended 

 with some difficulty, aud is a sort of bugbear to 

 microscopists. A few lines on the subject may not 

 he altogether unacceptable to the readers of Science 

 Gossip, or inappropriate to its pages ; more espe- 

 cially as one correspondent has already expressed a 

 desire for information on the subject. 



Perhaps the first thing to consider is the medium 

 best adapted for this purpose. 



Canada balsam and castor-oil seem to be the two 

 best media on account of their being little acted on 

 by the substances to be mounted iu them ; gelatine, 

 glycerine, and all aqueous media are, of course, quite 

 inadmissible from their solvent power. 



With regard to the salts themselves, the best 

 method of proceeding is to place a drop or two of a 

 solution of the salt, which it is desired to crystallize 

 for the purpose of mounting, on an ordinary glass 

 slide, allow it to dry slowly and completely, and 



then, if balsam is to be used, it must be applied in 

 a tolerably fluid state, the glass cover put on, and 

 the slide finished in the usual manner, with the ap- 

 plication of as little heat as possible. 



The use of castor-oil is attended with a little more 

 difficulty. A perfectly dry spot of crystals being- 

 selected, a small portion of castor-oil should be 

 dropped on gently, and allowed to insinuate itself 

 over every part of the spot to the total exclusion of 

 air bubbles. The thin glass cover must now be 

 carefully applied, all superfluous oil removed by 

 the application of bibulous paper, and the cover 

 sealed with shellac-varnish, during the drying of 

 which the slide must be kept in the horizontal posi- 

 tion. There are some substances which it is im- 

 possible to mount, such as chromic acid and per- 

 manganate of potash ; for, owing to their very power- 

 ful oxidizing properties, they become decomposed 

 in contact with organic matter. Crystals of such 

 compounds must therefore be prepared for observa- 

 tion extemporaneously, as, being very deliquescent, 

 they soon attract moisture from the air and become 

 liquid. 



There is, however, one salt specially worthy of 

 notice, which may be mounted with free access of 

 air, a thin glass cover to keep out the dust being all 

 that is required. It is the platino-cyanide of magne- 

 sium in its ordinary state ; the crystals are of a rich 

 red colour with bright green reflections. When 

 heated it becomes yellow from loss of water, and 

 then gradually absorbs moisture from the atmo- 

 sphere, and resumes its original red colour. A slide 

 of this, like many other objects, is, a " thing of 

 beauty," and a never-failing source of admiration ; 

 its gorgeous colours being heightened by the polari- 

 scope will call forth expressions of pleasure from 

 the enchanted beholder. Other salts may also be 

 enumerated as giving pleasing results, aud being 

 tolerably easy to mount. The sulphates of copper, 

 iron, magnesia, the double sulphate of nickel and 

 potash, the nitrates of soda and potash, chlorate of 

 potash ; among the organic bodies, saliciue, theine, 

 quinine, aud the other cinchona alkaloids. Aloine, 

 from aloes, either Socotrine or Barbadoes, may be 

 thus prepared. A fragment of either variety of 

 aloes being crushed on a glass slide, sufficient proof 

 spirit added to dissolve it, aud the solution covered 

 with thin glass, the arrangement set by that the 

 spirit may evaporate, the aloine will crystallize out 

 slowly (the slower the better). After a few days 

 fine crystals will be formed, which will require no 

 further preparation ; the glass cover may be fastened 

 down. This forms a very beautiful slide. 



There is also another beautiful salt called " Hera- 

 pathite." It is a salt of quinine ; its method of 

 preparation may be found in Hogg's work on the 

 microscope, and is too long for insertion here. This 

 salt requires great care in mounting, but forms a 

 nice object. 



