4i6 



KNOWLEDGE & SCIENTIFIC NEWS. 



[April, 1906. 



Conducted by F. Shillington Scales, b.a., f.r.m.s. 

 Elementary Photo-micrography. 



{Continued from page 392.) 



The details of adjustment and focussing may now be 

 recapitulated. The necessary centring adjustments 

 have been carefully explained, and the image may be 

 assumed to be projected and focussed upon the ground- 

 glass screen. It is well now to examine it carefully to 

 see that it is correctly in the centre of the screen, and 

 that its size corresponds with the plate to be used. If 

 a quarter plate is being used it is well to mark the limits 

 of the plate in pencil on the ground glass. Particular 

 care should be taken at this stage to note if the illu- 

 mination is even all over the field; if not, the sub-stage 

 condenser may need a slight re-adjustment by means of 

 its centring screws. It is also well to make now the 

 final adjustments of the iris diaphragm of the sub-stage 

 condenser, bearing always in mind that the apparent 

 sharpening of the image by closing the diaphragm is 

 more apparent than real, and that any excess in reduc- 

 tion of aperture will give fuzzy diffraction effects in the 

 photograph as well as prolong the exposure and in- 

 crease any tendency to unequal illumination. Of 

 course, the lower the numerical aperture of the objec- 

 tive, the smaller will be the cone of illumination that it 

 will bear, but it is a fairly safe general principle to shut 

 down the diaphragm cautiously and to stop as soon as, 

 or very soon after, the image begins to show signs of 

 reduction in brightness. All these adjustments can 

 only be carried out on the ground glass screen. The 

 plain glass screen is then put in place of the ground 

 glass, and the image is now invisible until the previously 

 adjusted focussing lens is laid lightly upon the former 

 screen; through this the eye looks directly down the 

 tube, the focussing lens performing as a positive ocular. 

 The increase in light is so marked that it will be 

 difficult to make any other adjustments than those for 

 the final focussing, and for this purpose the focussing 

 lens is moved about on the glass whilst one hand ad- 

 justs the fine adjustment of the microscope. It is then 

 only necessary to close the camera shutter gently, re- 

 place the plain glass screen with the dark slide, and 

 make the exposure. 



The extension of the camera gives considerable lati- 

 tude in magnification independent of any change in 

 objective or ocular, and, of course, with any change of 

 objective the sub-stage condenser will generally need 

 re-centring. But unless the camera is fairly short it 

 will be found impossible to keep the eye at the focussing 

 lens and at the same time to reach the fine adjustment. 

 It is, therefore, more than a convenience to have a light 

 rod running along the camera with a milled head at 

 one end and at the other a grooved pulley lying exactly 

 in line with the milled head of the fine adjustment of 

 the microscope, which is now nearly alwavs itself 

 grooved for this purpose, whilst between the two an 

 endless cord passes. The focussing adjustments can 

 then lae made with ease at the very end of a long camera 

 extension. This arrangement is better than any 

 Hooke's jointed rod. Messrs. Watson and Sons have, 



however, largely done away with the necessity for both 

 ground and plain glass screens (as well as focussing 

 rod), by inserting in their place a sheet of highly 

 glazed cardboard, which can be viewed directly from 

 the inside by means of a small door cut in the side of 

 the camera. The facilities this gives for ease of ad- 

 justment of all the centring movements is great, and 

 when I say that the flagella on a bacillus like Typhosus 

 can be readily focussed h\ this means it will be .seen 

 that it is amply sufticient for nearly all purposes. 1 

 may, perhaps, mention that I myself use this method 

 almost exclusively. But, of course, a good condenser 

 and a strong light, such as the arc light or the oxy- 

 hydrogen jet, are necessary, and for this reason I have 

 gone into so much detail on the older and more general 

 methods of focussing, \-c. 



(7'(i be cpucluded .) 



R^oyal Microscopica.! Society. 



At the meeting held on February 21, the President, 

 Dr. D. H. Scott, F.R.S., in the chair, .Mr. Waldron 

 Griffiths described his method for mounting delicate 

 vegetable tissues in xylol-balsam, and exhibited under 

 microscopes some excellent specimens of Tcziza, 

 Spirogyra in conjugation, and Zygnema. Mr. Beck 

 exhibited and described an optical bench for micro- 

 scope illumination, with ordinary or monochromatic 

 liLvht. Dr. Heblj exhibited a one-fifth inch objecti\e, 

 designed b}' Wenham in 1870, and made by Ross, to be 

 used either as a dry or water-immersion lens. Mr. 

 Walter Rosenhain described a new form of metallurgi- 

 cal microscope, the body of which is attached rigidly 

 to a specially-designed limlj carried on large trunnions, 

 the stage moving up and down as is usual in metallurgi- 

 cal microscopes, and being provided with coarse and 

 fine adjustments. Mr. Karhmd gave an abstract of a 

 paper by Mr. \\'. P. Dollman on ".V method of pro- 

 ducing stereo-photomicrographs," and exhibited a 

 number of exceedingly good stereoscopic prints in 

 illustration of the paper. Mr. Taverner then read a 

 short paper on " .'\ simple method of taking stereo- 

 photomicrographs, and of mounting the prints without 

 cutting." The methods described in the two papers 

 were quite different, and Mr. Dollman limits his opera- 

 tions to very low powers, giving amplifications of 9 

 to 20 diameters only. He uses a stop in front of the 

 objective and exposes first one side of the lens and 

 then the other, as he takes his two stereoscopic photo- 

 graphs. Mr Taverner uses higher powers, and a pecu- 

 liar stop at the back of the objective. The authors 

 adopt a similar arrangement for obviating the necessity 

 of cutting the prints. Mr. Rousselet gave an abstract 

 of a paper from the Hon. T. Kirkman, " -A second list 

 of Rotifers of Natal," in which a remarkable new 

 species, Copeus triangulatus, was described. 



Quekett Microscopical Club. 



At the annual general meeting held on February 16, the 

 President, Dr. F. J. Spitta in the chair, the annual 

 report showed an increase in membership during the 

 year, the number of members on December 31, 1905, 

 being 402. The hon. treasurer's report on the finan- 

 cial state of the Club was also very satisfactory. Dr. E. 

 J. Spitta, F.R..\.S., F.R.M.S., was re-elected president, 

 all the other officers were also re-elected, with the ex- 

 ception that Dr. G. C. Karop becomes a vice-president 

 in place of the late Mr. J. G. Waller. The president 

 delivered the annual address, dealing with " The Rela- 

 tive Merits of the Short and Long Tube for Micro- 

 scopes." The short-tube stand was probably intro- 



