KNOWLEDGE & SCIENTIFIC NEWS. 



[October, 1905. 



Conducted by F. Shillingto.n bcAi.Es, f.r.m.s. 



Limit of Visibility of IsolaLted Elements 

 in the Micioscope. 



K. Strehl has made sonic interesting ubi.ervations in 

 connection with the recent work ol Siedenlopl and Zsig- 

 mondy, whose speculations as to the visibility of ultra- 

 microscopic particles he reg-ards as partly hypothetical, 

 partly not Iree from other objections, and considers 

 their actual results as of most importance. With the 

 most intense sunlight with an illuminating system of 

 N..A. 0.3, and an observation system of N.A. 1.2, used 

 with strong oculars, the least value they obtained for 

 the edge of their cube-shaped gold particles was 4^/j. 

 (= .000004 m.m.) for bright spots on a dark ground. 

 For dark spots on a bright ground, on the basis of the 

 diffraction theory, with N.A. 1.5., wave-length 500 /x/t. 

 eye-sensitiveness limit 5 p.c, and a completely aberra- 

 tion free pencil, Herr Strehl himself has demonstrated 

 the following limits of visibility : — Smallest diameter of 

 round dark apertures, self-luminous 48 mm-, illuminated 

 34-5 MM-, and smallest breadth of straight dark slits, 

 self-luminous 10.5 fi/j.., illuminated 2.5 ^^. The com- 

 parison of both methods of observation, as well as the 

 results, has importance in connection with the investi- 

 gation after ultra-microscopic bacteria. Compare the 

 original statement in Central ZeiL f. Optik. u. Mech. 

 xxvi. (1905;, p. 117, :uid J.R..M.S. (1905), p. 521. 



Imbedding with Incomplete Dehydration. 



\V. J. v. Osterhout, Univ. California, Pub. Bot., 

 and J.K.M.S. (1905), p. 526, recommends the use of a 

 saponaceous medium for imbedding vegetable tissues 

 instead of parallin, namely, cocoanut oil and sodium 

 hydrate nii.\cd in the proportion of 70 c. cm. of oil to 

 38.5 a cm. of 28 per cent, solution of KHO in water. 

 ITie oil is wju-med in a water-bath, and the lye added 

 gradually, the mass Ijeing stirred meanwhile. Tlie 

 tissue to be imbedded is warmed in a water-bath, arid 

 the soap added as long as it will dissolve. 'Ilie whole is 

 poured into a suitable receptacle until sulBcienlly firm 

 to cut into blocks. The birjcks are then treated as in 

 the para din methixl. Perfect sections, one micron 

 thick and several feet long, are easily obtained, llie 

 sections may be treated in the usual way, either by 

 making them adhere at once to slides, or by first dis- 

 solving out the soap by soaking them in water. If 

 required to be fi.xed to slides in serial order, they aro 

 placed on slides previously coaled with while of egg 

 and then dried ; they are then moistened with xylene, 

 w-hich spreads them out, and makes them adhere. A 

 piece of absorbent muslin is next pressed gently on the 

 sections, and when the xylene has evaporated, the 

 muslin is moistened with water. TTie slide is Mien 

 heated to coagulate the idbumen, and fix the sections 

 to the slide. The muslin is nf>w moistened again, :md 

 afterwards carefully removed, after which the sections 

 can be treated as usual. .Mcohol may Ije used instead 

 of water for imbedding by this nK-thcxI. The tissue 

 partly dehydrated is placed' in alcohol on a water-bath, 

 and soap added till no more will dissolve. 



Microscopical Lectures. 



The Manchester Microscopical Society have ag.iin 

 sent me their annual prospectus of lectures for the 

 coming winter, which are given by members of the 

 Society, for the most part gratuitously, save for out- 

 of-pocket expenses, in the districts around Manchesler, 

 and even in the Xorth-W'est Riding of Yorkshire and 

 the Western Counties. The list of lectures numbers 

 55, and of lecturers 19, and the lectures are given at 

 meetings of any society, science club, mechanics' in- 

 stitute, etc., which applies for them. Practical demon- 

 strations in microscopy, microscopical exhibitions, and 

 the moimting of microscopic objects are also gi\on if 

 required. So excellent a scheme deserves the highest 

 commendation, and might, with advantage, be imitated 

 by other societies. In particular, it seems to me that 

 the Quekett Club could well extend its usefulness by 

 adopting such a scheme as this. It would bring micro- 

 scopical matters before a larger public, would dissemi- 

 nate scientific knowledge, could not fail to increase the 

 numbers of those who are interested in microscopical 

 matters, and, incidentally, would make known the work 

 of the club, and bring it new members. What the 

 Manchester Microscopical Society can do in the popu- 

 lous districts in and around Miuichester could surely 

 not fail to be e\cn more successful in the densely popu- 

 lated district of London. 



Glycerine a.s Bl Mounting Medium. 



The use of glycerine as a mounting medium, con- 

 venient as it is in many respects, has several disad- 

 vantages. Pure glycerine has ; refractive index of 

 1.46, but, by diluting it with an equal quantity of water, 

 the refractive index is lowered to 1.4, and thus the 

 visibility of many structures is increased. It is im- 

 portant, however, that the object should be thoroughly 

 impregnated witli glycerine, and a fruitful cause of diffi- 

 culty is the presence of air bubbles in the tissues ; such 

 air bubbles, unlike those in objects mounted in Canada 

 balsam, not being subsequently absorbed. 'Hie essen- 

 tial difference between mounting in Canada balsam and 

 mounting in glycerine is that, whereas objects mounted 

 in the former medium must be thoroughly dehydrated 

 — that is, freed from every trace of water — objects 

 mounted in the latter medium must be mounted direct 

 from water only. It is advisable, therefore, to soak 

 the object carefully in water, and to use water that has 

 been recently boiled, to get rid of any air in it. /\fter 

 this, the object must be well soaked in glycerine until 

 every part of it is thoroughly impregnated. Glycerine 

 jelly contains gelatine, and requires to be melted before 

 use, after which it sets again ; Farrant"s medium con- 

 tains gum arable, and sets at the edges ; but glycerine 

 itself not only does not set, but is so hygroscopic as to 

 absorb water readily from the air. The mounts must, 

 therefore, be enclosed in some way — ringed, as it is 

 termed. The usual way is to centre the slide on a 

 turntable, and ring the cover-glass with a thin circle of 

 melted glycerine jelly, and, after this has set, to ring 

 again with one or two coats of gold size, llic gold 

 size must not be too thin ; in fact, it .should just be thin 

 enough to leave the brush easily, and no more. Any 

 other cement, such as zinc white or Urunswick black, 

 may then be applied on the top of the gold size. 

 Another, and less-knf)wn methfKl, is to ring in the same 

 way with Canada balsam instead of glycerine jelly. 

 The balsam, likewise, should be comparatively thick. 

 It is important, however, that both cover-£,'lass and 

 slide, beyond the border of the former, should be quite 



