222 



SCIENCE-GOSSIP. 



Mediums for Mounting. — Twenty years ago, or 

 thereabouts, when I was working at microscopy, a 

 difficulty was experienced in finding a medium 

 wherein certain objects, such, for instance, as palates 

 of moUusca, would not be completely overpowered, 

 as it were, with light ; that is, were made so trans- 

 parent that their proper form could not be distin- 

 guished when magnified. Canada balsam was then 

 almost only used ; and gum dammar, which made 

 matters worse, was just beginning to come into 

 fashion. The general law of microscope vision that 

 " an object becomes more distinctly visible the more 

 its refractive power differs from that of the medium in 

 which it is mounted " was then, if not actually for- 

 mulated, at least practically recognized by mounters, 

 professional or otherwise. In most cases there was 

 no other resource but to mount the object dry, 

 which of all other means is at once the most diffi- 

 cult and the most unsatisfactory. Judge then of 

 the delight which thrilled the microscopical worker 

 when somebody in Belgium (I think it was Dr. 

 Van Heurckj proposed the use of gum styrax as a 

 mounting medium in connection more especially 

 with diatoms. Its index of refraction when pure is 

 1-6 (Canada balsam is only 1-540), and that of the 

 silica of diatoms, etc., is i'43, so that the difference 

 is pretty wide. The principal value and strength 

 of styrax as a mounting medium consist, so I have 

 always thought, in enabling objects to be mounted 

 therein which previously had to be mounted dry. 

 Thus palates of moUusca, scales of butterflies 

 and moths, various anatomical and physiological 

 objects could now be seen with a clearness and 

 convenience never before attained. I never saw 

 anything more approaching what might be termed 

 micro-real vision than a piece of broad-leaved 

 meadow-grass which had been prepared by mace- 

 rating in water, alcohol and benzine, successively 

 for some days ; and then mounted in styrax and 

 viewed with its epidermis carefully focussed under 

 a French ^-inch objective of N.A. 0-64, the con- 

 denser used being the Abbe of N.A. 14 with the top 

 lens removed and a piece of optically-worked blue 

 glass placed beneath the diaphragm. Styrax is 

 recommended also as a medium for wood sections, 

 insects and the like ; also for rendering \'isible the 

 nucleus of vegetable cells previously stained with 

 haemotoxyn. I have found it exceedingly useful 

 for displaying the bordered pits of coniferas in 

 radial section after staining in logwood ; in fact, I 

 know of no other medium which shows them as 

 well. For some other objects, however, it is use- 

 less ; for example, striated muscle. If you cannot 

 mount muscle in glycerine, by all means use a 

 mixture formed by dissolving "fiddlers' resin" in 

 oil of bergamot, v/hich is about the least refractive 

 of all mounting media : this will enable you to see 

 the striae almost as well as in glycerine. A solution 

 of fiddlers' resin in oil of juniper, filtered through 

 cotton-wool stufifed loosely in the neck of a glass 

 funnel, is an excellent medium for many purposes ; 

 it has a pasty flow which makes it work very 

 pleasantly, keeps perfectly limpid, and gives very 



good definition. A considerable "puff" or a big 

 "fillip" came over from the United States not 

 long ago. A certain Dr. Edwards, of Newark, N.J., 

 in a note to Science-Gossip, extolled the merits of 

 GunThis as a medium for diatoms, etc. — I have 

 not as yet seen this article for sale in the catalogue 

 of any London dealer, but I have seen it in those 

 of two north-country dealers, as sold dissolved in 

 bisulphide of carbon. Has any reader ever tried 

 it ? If so, let us know what is thought of it, for the 

 benefit of science. Some twenty years ago a tough 

 and hale out-door naturalist of the old school 

 admonished me to beware of mounting in Canada 

 balsam delicate spicules of organic carbonate of 

 lime, such as are found in the Echinodermata. I 

 have never forgotten the advice. Even some very 

 delicate dermal plates of Ciicumaria drummojidi, 

 which I mounted twenty years ago in dammar 

 solution (not nearly so corrosively acid as Canada 

 balsamj are just now beginning to go — the lime 

 is gradually being eaten away, leaving, however, 

 the pure organic matter of the same form and 

 pattern as before, though of course much more 

 transparent, being a warning that the ideal 

 medium for mounting is still to be discovered. 

 — [Dr."] P. Q. Keegan, Patterdale, Westmoreland ; Sep- 

 tember, 1896. 



Preparing Hydra for the Microscope. — To 

 kill Hydra in an extended condition does not appear 

 so easy as the text-books would lead one to expect. 

 One biological authority states that it may be 

 killed" in a fairly extended condition" by the 

 slow addition of alcohol to the water containing it, 

 but in my hands the results by this method have 

 never been worth the trouble of permanent 

 preservation, and bear but little resemblance to the 

 living organism. Another microtomist writes that 

 it is easily killed in an extended condition by 

 running a drop of osmic acid under the cover-glass. 

 This method also, after patient trial, failed to give 

 me anything approaching a resemblance to life, 

 though useful enough when only sections were 

 wanted. A few weeks ago, ha\ing made a great 

 haul of Hydra on a pond-hunting excursion, I 

 decided to try a number of narcotizing agents in 

 the hope of discovering one that would give me the 

 animal in very nearly the extended state it 

 assumes in life. Hydrochlorate of cocaine, so 

 valuable with the Hydrozoa was useless, however 

 cautiously applied, the Hydra gradually contracting 

 as the narcotizing proceeded. Chloroform at first 

 seemed promising, but when it had associated 

 sufficiently with the water to reach the animals 

 they rapidly commenced to retract. Chloral 

 hydrate simply induced maceration, though giving 

 fairly extended specimens. At last mono-bromide of 

 camphor was cautiously applied to a specimen in 

 an extreme state of extension, and with pleasure I 

 noted that narcotization was being effected without 

 the least contraction of the tentacles. When 

 irritation failed to produce retraction the specimen 

 was killed and fixed with osmic acid, and a perfect 

 mount was obtained after several dozen failures 

 with all kinds of re-agents. Mono-bromide of 

 camphor is but sparingly soluble in water, and 

 the first attempt made by adding a crystal to the 

 water containing the Hydra was fruitless owing to 

 the length of time the drug occupied in dissolving. 

 A saturated solution, made b}- boiling crystals in a 

 test-tube, is the best method of use, a little of the 

 solution being added to the water containing the 

 organisms to be narcotized — George T. Harris, 33, 

 Lindore Road, Wa?idszivrth. 



