NOVKMBER, 1901.] 



KNOWLEDGE 



261 



microscope has done lUiring recent years should not have 

 received any treatment whatever in so exhaustive a work as the 

 new edition of " Carpenter on the Microscope.' It is a liighly 

 technical and important subject, and has become an absohito 

 essential in all iron and steel works. Tn fact, there is probably 

 no factory of standing that is not e(iuii)pod with suitable 

 instruments both for observing and i>hotogiaphing. 



Although much information regarding the chemical constitu- 

 tion of the metil is disclosed by the microscope, it is in 

 ascertaining the mechanicil properties that the special value of 

 the examination lies. 



For instance, the strncture of steel varies with the different 

 degrees of hardness and the amount of heat to which it has 

 been subjected, and it is possible to gain definite information 

 concerning the suitability of the metal for the purposes for 

 which it is to be used by means of the microscojje. 



In the manufacture of guns the microscope is invariably 

 resorted to, and it can be definitely determined before the 

 manufacture is proceeded with whether the metal is suitable for 

 the purpose, or any defect has taken place in the heating or 

 iiuenching which would render the gun unsafe or unsatisfactory. 



Engineers can detect flaws, blow-holes, defective welds, etc., 

 and in many ways are able in an early stage to avoid the trouble 

 incident to the use of imperfect metal in the finished article. 

 Microscopes for the exclusive purpose require no substage 

 apparatus or mirror, and although thecom|)lote stand is usually 

 employed, the demand has become sufficiently marked for makers 

 to produce special instruments in which a large mechanical stage 

 is provided, but no fittings beneath the stage. Among these 

 are Reichert, of Vienna, C. Baker, of London, and (^ueen, of 

 Philadelphia. In all of these the vertical illuminator plays an 

 important part : some observers preferring the pattern with a 

 prism, others the cover-glass reflector, while many employ both, 

 finding that each pattern is advantageous according to the 

 structure examined. 



When the light and illuminator have once been adjusted, it 

 is important that no movement of the body of the microscope 

 takes place, or the illuminating would have to be re-set. To 

 obviate this, these special microscopes by Queen and Baker have 

 rackwork focussing adjustment to the stage, together with 

 levelling screws, so that any want of parallelism in the piece of 

 metal may be corrected, and the face set at right angles to the 

 plane of the objective. It will be seen from this that the 

 subject has received careful consideration, and that suitable 

 means are available for accurate work. 



The metals themselves that are to be observed have to be 

 prepared with great care ; the processes are technical, and vary 

 with the purpose of the examination. Generally a small sample 

 block is taken, and cemented to a piece of glass. It is then 

 ground and polished on a series of emery papers, and finally 

 with a very fine polishing material such as rouge, until no 

 scratches can be detected. 



Although the harder portions can be in manj' instances seen 

 at this stage, chemical means have to be resorted to, to 

 differentiate tho structure, the action being unequal on the 

 different constituents, and it is by the treatment with such 

 chemicals — nitric acid and liquorice juice being amongst the 

 most important — that the chemical composition is detected. 

 Many of the constituents have received distinctive names, among 

 them being : — 



Ferrite, which is the name given to pure iron. 



Cementite, representing the iron carbide in steel. 



Pearlite, a mixture of cementite and ferrite. 



Martensite, the structure of quenched steel. 



Austenite, a name given to structure which is produced in 

 steel in which there is a high proportion of carbon. 



There is vastly more to be learned of this interesting subject, 

 and it is a field in which the amateur could with interest and 

 profit make investigations. 



A very full paper on the matter appears in the / htslmled 

 Aiinnal oj Microscopy nf 1900, by Mr. W. H. Merrett, to 

 which I have referred for several of the foregoing details. 



McLTiPr.rF.D Images im Diatoks. — Happening to hold a 

 needle over the mirror of the microscope while a slide of 

 Triceratium was under examination, I noticed that its point 



appeared dimly in each of the interspaces, and on racking the 

 bt)dy of the niicrosco]ie slightly upwards in the same manner as 

 is necessjiry when observing multiplied images in the cornea of 

 the oj-e of a beetle, I noticed that it became distinctly sharp. 

 I then placed a piece of black pajier cut in the shape of a cross 

 on the mirror, and with a ,^ in. objective saw it sharply displayed, 

 although on a much smaller scale than in the eye of beetle. 



Experiments were then made with (Joscinoiliscus and other 

 diatoms, and the same result was shown. This would seem to 

 indicate that these interspaces are li'iiticular in shape, and it 

 maj" be that with this knowledge further light may be shed on 

 the complex (piestion of the ultimate structure of some of the 

 Diatomaceaj. 



FitKi: Dkmonstrations ix JIicro. Maxiiti.atiox.— 0. 

 Bakkr's 1'.I02 Cai'AI.oi-.ii'.— The newly issued edition of 

 C. Baker's catalogue for the present season has been sent to me. 

 The system of classification into separate parts, of microscopes 

 and their immediate accessories, ajiparatus for collecting, 

 bacteriological and blood examination apparatus, microscope 

 specimens, etc., which made the ])revious issue of this firm's 

 catalogue so useful for reference purposes, prevails in this 

 new issue. 



A])art from the very clear descrijition of the many accessories 

 which are required by the modern student, amateur or 

 laboratory worker, there are one or two features of interest in 

 their list which call for special notice on the part of micro- 

 scopists generally. 



On the first and third Friday and the second and fourth 

 Tuesday in each month, from October to Juno, between the 

 hours of '■') p.m. and li p.m., demonstrations will be given of 

 microscopical manipulation. A syllabus has been prepared, and 

 a series of six demonstrations is arranged to cover practical 

 working with the microscope with modern accessories and by 

 the latest methods. 



I am not aware that any such provision has ever been made 

 before, or classes held where such instruction as is now offered 

 by Messrs. C. Baker could be obtained. It is particularly 

 stated that " these demonstrations will be free to all, no charge 

 of any kind whatsoever will bo made, and no obligation to 

 purchase is incurred by those availing themselves of this offer." 



Another feature is the development of their Slide Lending 

 Department, a system which, T believe, originated with this 

 firm. They have had pamphlets and descriptions prepared to 

 accompany the various slides which are had on loan from them. 

 These have been written by experts in the various subjects, and 

 should be of great value to the private worker. 



Catalogues of scientific apparatus nowadays are not so much 

 mere price lists, but invariably contain a large amount of useful 

 information and data, and are always handy for reference 

 purposes. 



This new catalogue is one that will be found especially 

 practical, and microscopists should possess themselves of one. 



NOTES AND QUERIES. 



J. U. }rnrrh. — There is no occasion to use balsam and shellac 

 for fixing your rock sections to a glass plate during grinding. 

 Canada balsam answers ])erfectly well if only it is used with 

 care. The secret lies in the proper heating of the Canada 

 balsam ; if this is carried too far, it easily chips, and if not 

 sufficiently it will not harden quickly. If you make further 

 experiments, you will probably be more successful. There 

 should be no air bubbles between the rock and the glass to 

 which it is fixed ; in the event of these appearing the process 

 must be repeated. Balsam and shellac is used by optical glass 

 workers, but as you would only want a very small quantity 

 you could quickly ascertain suitable proportions : very little 

 shellac would be needed. 



ir. If. Pnirtiiill, — The powers and numerical apertures of the 

 Reichert objectives Xos. 3 and 6 are respectively J" -'M, and 

 i^" '85. These are first-rate lenses. The stand you mention is a 

 very poor one, and yon certainly cannot get the full benefit 

 derivable from the objectives with such mechanical means. 

 Swift's four-legged stand is a very good one and is conveniently 

 portable. I do not think you could get greater value for your 

 money from any other maker, and in my oinnion this stand 

 possesses many advantages over those made abroad and is 



