20 



KNOWLEDGE. 



[January, 1901. 



Conducted by F. Shilltngton Scales, f.e.m.s. 



MICROSCOPICAL RESOLUTION. 



For the meeting of the Royal Microscopical Society on the 

 19th November, a paper had been announced by Prof. Everett, 

 F.R.S., dealing with " Microscopical Resolution." Those 

 interested in the theory of the microscope who attended in the 

 hope of increasing their knowledge of this aspect of the 

 subject were doomed to be disappointed as far as this paper was 

 concerned, for the learned author had evidently misapprehended 

 the scope of Equation .32 of Lord Rayleigh's paper on " The 

 Theory of Optical Images," recently reprinted in the R.M.S. 

 Journal. The elementary formula which Prof. Everett deduced 

 in the usual elementary way for the difference of phase between 

 adjoining slits of a grating — and which Lord Rayleigh gives as 

 No. 45 — is quite correct, and leads, when discussed in a similarly 

 elementary manner, to the familiar diffraction spectra, but 

 without disclosing the not entirely unimportant intermediate 

 secondary maxima. But Lord Rayleigh's paper goes far beyond 

 this ; it determines the distrihution of light in the final image of 

 a grating, with only these two simplifying assumptions : that 

 the number of lines is infinite, and that the lines are negligibly 

 narrow compared to the dark intervals. 



The theorists were, however, fortunate in hearing an ex- 

 position from Dr. Johnstone Stoney, F.R.s. — a rare visitor— who 

 was asked to speak. 



Having pointed out that the familiar but elusive " rays " of 

 light can be used only for elementary purposes, and must be 

 supplanted by " waves " in all thorough investigations, and 

 having alluded to his method of resolving undulations into 

 plane wavelets. Dr. Stoney proceeded to communicate some 

 extremely interesting results of his experiments with 

 gratings. 



He first showed how tuxi lines a certain distance apart could 

 be resolved by an aperture quite incapable of resolving a greater 

 number of Imes at similar distances. But whilst the lines 

 are properly resolved— that is, separated by a dark interval— 

 their distance apart is, in these circumstances, misrepresented. 

 They appear too far apart in inverse proportion to the distance 

 apart of the portions of two diffused diffraction fringes utUized. 

 This is an experimental proof of the correctness of the 

 reasoning leading to the Abbe theory, which is as novel as it is 

 important. 



His next point was equally interesting and valuable. When 

 the number of lines in a grating is finite, and particularly when 

 it is small, the complete diffraction spectrum produced by the 

 grating is not hmited to the familiar principal maxima ; there 

 are (n-2)— n being the number of lines in the grating — secondary 

 maxima between every two principal ones, and those of these 

 secondary maxima which are near a principal maximum are of 

 appreciable brightness. Dr. Stoney has been able to demonstrate 

 that these secondary maxima, when combined with the direct 

 light, (Ac first difractii/n-spectrum itself being excluded hy 

 reducing the aperture of the nucroscope, are capable of giving a 

 feeble kind of resolution. And, as in the previous case, there 

 is again complete agreement between the results of the direct 

 experiment and that to be expected theoretically, for the faint 

 image secured in this way shows the exact defects and 

 peculiarities which theory demands. 



Dr. Stoney's third point also proved of interest. Perhaps the 

 greatest defect of the published accounts of the Abbe theory 

 lies in the utter want of definite information of a practical kind. 

 It is stated that there can be no complete similarity between 

 object and image unless every diffraction spectrum of appreciable 



intensity is utilised, and api)arently with a view to impressing 

 the confiding microscopist with the importance of this doctrine, 

 certain experiments with the " Diffractions-platte " are mentioned 

 which yield a dissimilarity between object and image that is 

 absolutely startling. Of course, this is right when complete 

 similarity is taken in its strictest sense, i.e., down to the minutest 

 detail, which, however, no practical man would expect to see 

 under any conditions. What the latter desires to know is how 

 )inirh dissimilarity he must be prepared for, and to the ])ractical 

 man Dr. Stoney's testimony as to the remarkable improvement 

 of microscopical images when the second spectrum is admitted 

 must, therefore, be a welcome guide. 



Dr. Stoney proceeded to make some further remarks on the 

 importance of Condenser-adjustment in attempting very delicate 

 resolving tests, but his interesting communication had to be 

 terminated with a view to securing the remaining portion of the 

 evening for another paper on the agenda. 



Very few microsco])ists are really competent to appreciate the 

 value of microscopical theory, and the high importance of taking 

 every advantage that can be suggested for accurate manipulation. 

 It is to be hoped, therefore, that Dr. Stoney's suggestions may, 

 in due course, appear in print, and thus afford an opportunity 

 to intelligent and thoughtful workers to assess them at their 

 proper value. Dr. Stoney's remarks were not only of interest 

 but to the point, for microscopical resolution was the subject 

 which was opened by the somewhat disappointing paper which 

 had brought our veteran physicist to this meeting. 



QuEKETT Microscopical Club. — The 408th ordinary meeting 

 was held on November 20th, at 20, Hanover Square, W., the 

 Vice-President, J. G. Waller, Esq., F.s.A., in the chair. There 

 was as usual a large attendance of members and visitors. Mr. 

 W. H. Langton exhibited a small portable microscope, which he 

 had constructed without the use of a lathe. It was fitted with 

 sliding coarse adjustment, two-speed fine adjustment, and 

 motions to stage, substage and mirror. The various adjustable 

 parts were kept in alignment with the body of the instrument 

 by means of grooves in the ring fittings, the grooved rings 

 travelling on a steel wire fixed in alignment with the body tube. 

 Mr. Langton was complimented by many members on the 

 ingenuity displayed in the construction of the instrument. 



Mr. W. Wesch^ gave a demonstration, illustrated by the 

 lantern, of the homology of the mouth partsof Dipterous flies with 

 the mouth of the cockroach. It was shown how the mandibles 

 were fused into the upper part of the proboscis of the blow-fly, 

 and the maxillae, or inner jaws, embedded in the base. Mr. 

 Weschc' also exhibited a number of minute palpi discovered by 

 himself in many different species of Diptera. 



Mr. L. R. Gleason gave an address on bacteriology as 

 considered from the point of view of the amateur. It was 

 illustrated by lantern slides of cultures and apparatus, and by 

 specimens under the microscope. He wished to correct the 

 popular idea that very high powers and expensive apparatus 

 were a sine qua non for bacteriological work. A great deal could 

 be done with a |th-inch objective and a little ingenuity in the 

 preparation of apparatus. He would not, of course, recommend 

 the amateur to undertake the culture of pathogenic germs, but 

 the non-pathogenic germs were quite as interesting to study, 

 and, moreover, were in many cases of the highest value to man. 

 Linen, hemp, tobacco, opium, butter, cream, cheese, and a host 

 of other domestic products were produced by the action of these 

 invisible workers, and he trusted that many microscopists in 

 search of a field for study would turn their efforts in this 

 direction. 



"Journal op the Quekett Microscopical Club."— The 

 half-yearly number of this journal has just reached me, and 

 contains several interesting and useful papers, amongst which I 

 may mention Messrs. Marks and Wesche's " Further Observa- 

 tions on Male Rotifers," and a second part of Mr. D. J. Scour- 

 field's " Synopsis of the well-known species of British Fresh- 

 water Entomostraca," dealing with the Copepoda. Mr. R. T. 

 Lewis contributes a note on a hitherto undescribed species of 

 Chelifer, illustrated by a plate ; and Mr. D. Bryce has a note 

 on two new species of Philodina. Among the more popular 

 articles may be mentioned Mr. W. H. Harris' " Remarks on 

 the Emission of Musical Notes, and on the Hovering Habit of 

 Eristalis tenux" ; and amongst practical notes one by Mr. H. J, 



