March, igo5.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



393 



Microscopical Society on Xovember ig, 1890, Professor 

 Bell exhibited a print of the original photograph, which 

 he (the present lecturerj had roughly copied in his note- 

 book, and which showed the letter R the right way 

 roimd and not rev<'rsed as in Carpenter. 



Barnes Dissecting Microscope- 

 Messrs. .\. !■;. Stalry and ( 'o. ha\c recently brought 

 to iny notice a simple dissecting microscope made by 

 the Bausch and Lomb Optical Co. , which is both practi- 

 cal, serviceable, and inexpensive. The body is of neatly 

 finished light wood shaped to form hand rests. The 

 stage is a glass plate, easily removable for cleaning. 

 The mirror is as large as the stage, giving effective 

 illumination. A black or white metal plate can be 

 laid over the mirror when a black or white background 

 is desired. The lens carrier is adjustable for focussing. 

 The base forms a hinged wooden box with receptacles 

 for magnifiers, tweezers, dissecting needles, &c. The 

 whole stand, including an inch doublet lens with large 

 flat field and good definition, costs onlv half a guinea. 



(L,C( 



Notes and Queries. 



.1., //;///. — I have put your query before a gentleman in 

 Cambridge who is an authority on the Carboniferous Rocks, 

 &c., and he considers that Harker would be most suitable for 

 your purpose, but as you say you have this and it is not quite 

 what you want, he suggests that you should endeavour to see 

 Hatch's " Introduction to the Study of Petrology " and " Text- 

 book of Petrology "; Geikie's " Structural and Field Geology," 

 and Rutley's " Study of Rocks," and see if one or more of 

 these will help you. Cole's " .•Mds to Practical Geology " is 

 also useful. Of course, you will be aware, that the recog- 

 nition of the constituents of rocks cannot be gained from 

 pictures in books without a preliminary broad and general 

 study of the subject. There seem to 'be no books dealing 

 e.xclusively with the special subject in the way you require. I 

 regret the oversight to which you call my attention. 



D. B., MaiichcsUi'.—l think both the books you mention are 

 very suitable for your purpose. Strasburger is perhaps a little 

 better than Bower for anyone working alone and unaided. I 

 think you would find Watson's " Praxis " microscope satis- 

 factory for all kinds of work, and would suggest vour ob- 

 taining with it :■; and i inch objectives (Parachromatic 

 series), the i^ being more generally useful than the A inch, 

 and the ;| having a N.A. of -87, and a No. 2 ocular.' This 

 would cost you, with case, £6 los., but I strongly recommend 

 your adding a " Scop " .Abbe Condenser, with iris diaphragm, 

 and an extra No. 4 eyepiece at a total cost of j[y 1 2s. 6d. 



C. C. Dobell and C. IV. Archer.— I think the "ultra-micro- 



scope " to which you refer must be an allusion to Dr. Sieden- 

 toff and Dr. 2sigmondy's apparatus for the making visible of 

 " ultra-microscopic particles." Vou will find an accomit of 

 this in the "Science Year Book" for 1905, article "Micro- 

 scopy," page 117, with further remarks in the issue for igo6, 

 page 107; also in the Journal of the Royal Microscopical 

 Society for October, 1903. The principle in brief is the 

 focussing of an arc-light upon a small spectroscopic slit and 

 condenser so as to pass a narrow beam of light at right angles 

 to the optic axis of the microscope upon the particles to be 

 made visible. According to Lord Rayleigh, there is nothing 

 except lack of light to hinder the visibility of any object, how- 

 ever small ; but if its dimensions are much less than half a 

 wave-length, its apparent width will be illusory. In other 

 words, the existence of the object can be shown if only the 

 light be strong enough, but this will be no guide to its real 

 appearance. It may be of interest to add that, accepting the 

 size of a medium size molecule as 6^;i, a specific intensity of 

 luminosity considerably exceeding the power of the sun's rays 

 would be necessary to discern such molecules. It is advisable 

 to lay particular stress upon the important difference between 

 the mere making visible of an object and the seeing of it as it 

 really is, as I find many workers with the microscope failing 

 to realise this distinction, and thus entertaining erroneous 

 expectations. 



T. H. Russell, Edj^bastan. — The only way to find out the 

 magnification of eyepiece and ocular is to measure such 

 magnification, and the easiest way is to project the image of 

 a micrometer at a distance, as explained in Carpenter and 

 other books on the microscope, and to make the necessary 

 simple calculation. If you project the image of the micro- 

 meter on a sheet of paper at ten inches from the eye-lens of 

 the ocular you will obtain a similar, if less accurate, result, 

 but the distance }nust be fen inches if it is to correspond 

 pretty closely with the magnification as seen visually through 

 the uiicroscope, because ten inches is the normal visual dis- 

 tance. If your paper is fiu"ther away the magnification is 

 proportionately greater, and no longer corresponds to that 

 given when you look dowu the microscope tube. It must be 

 evident that alteration of the tube-length will alter the magni- 

 fication visually, and in just the same way upon the paper, but 

 the measurement is always taken at ten inches from the tube 

 length because that remains the visual distance of the eye 

 whatever the tube length. .Vdjust the tube length as you will, 

 you must always measure the projected image ten inches away, 

 or, if [Hore, you must reduce the result to what it would be at 

 ten inches. I hope this is quite clear now. 



Rev. ]. B. Williams. Exniinster. — Paraffin of various melting 

 points is used for embedding and infiltrating specimens, one 

 melting at about 50° C. being perhaps the most generally use- 

 ful in a room temperature of about 16^ C. With rocking 

 microtomes a somewhat harder paraffin is better. Hard 

 objects naturally require a harder paraffin than soft ones. 

 Paraffin of various melting points can be readily obtained, 

 but if your paraffin is too hard, you had better mix it with 

 some of a lower melting point, rather than with any other 

 substances. The paraffin in a section can be got rid of by 

 soaking in .xylol. Tissues are dehydrated with alcohols of 

 variously graduated strengths, s\ich as 30 per cent., 50 per cent., 

 75 per cent., and g6 per cent., taking care that sufficient time 

 is allowed in each. Before embedding, the object must be 

 soaked for a considerable time in cedar oil or clove oil. .Vfter 

 the paralfin has been got rid of, you can proceed to stain at 

 once in alcoholic stains, but if watery stains are used the object 

 nnist be first passed back through the alcohols in reverse 

 order, and then before mounting in Canada balsam the de- 

 hydration must be gone through again, finishing with a final 

 soaking in xylol, cedar oil, clove oil, &c. The best all-round 

 stains for vegetable tissues are ha;matoxylin, borax carmine 

 (3 per cent, carmine to 4 per cent, borax in 70 per cent, 

 alcohol, a concentrated solution being first made and after- 

 wards diluted with an equal amount of 70 per cent, alcohol), 

 methyl blue, and methyl green. \'egetable tissues ni'ist be 

 first hardened for some days in methylated spirit. Cross and 

 Cole's "Modern Microscopy" (4S.i contains very clear instruc- 

 tions for elementary mounting, and my own " Elementary 

 Microscopy " (3s.) has also a chapter on the subject. 



{Communications and Enquiries on Microscopical matters should be 

 addressed to F. Shitlington Scales, "Jersey," St. Barnabas Road, 

 Cambridge.'i 



