346 



SCIENCE-GOSSIP. 



MICROSCOPY FOR BEGINNERS. 

 Bv F. Shillington Scales, F.R.M.S. 



(Continued from fiagc 314.) 



The cone of light transmitted by the condenser 

 should approximate to that of the objective ; but few 

 objectives will stand solid cones of light equal to 

 their own apertures. If the worker is using the 

 Abbe illuminator of 1-2 N.A., he will, of course, 

 only obtain a total aperture of 1 N.A., unless the 

 condenser is in immersion contact with the under 

 side of the slide. Theoretically, therefore, this con- 

 denser is suitable for lenses of I N.A. and even 

 more, and suitable also for lenses of lower apertures 

 by the simple process of stopping the aperture down 

 by means of the iris or other diaphragm. The 

 worker soon becomes expert in the use of the iris 

 diaphragm ; but we would caution beginners against 

 the too prevalent tendency to cut off an unnecessary 

 portion of the marginal rays in order to secure 

 contrast in the image. By so doing, the latter suffers 

 rather than gains, diffraction images being set up. 

 As a matter of fact, the diaphragm should be closed 

 gently and cautiously, just enough to slightly ac- 

 centuate the image and no more ; and it will generally 

 be found that, under these circumstances, if the eye- 

 piece be removed, the back of the objective, examined 

 down the empty tube, is found to be from § to J 

 filled with light. A good objective should stand 

 satisfactorily what is accordingly often spoken of as a 

 J cone of illumination. 



In most cases strictly axial, i.e. central, illumina- 

 tion is used ; but there are certain cases, such, for 

 instance, as the structure of diatoms, for which 

 oblique illumination is necessary. Formerly this was 

 obtained by swinging the mirror, or mirror and con- 

 denser combined, slightly to one side, but more 

 generally now by the insertion of a stop in the 

 carrier beneath the condenser. In this stop a narrow 

 slot has been cut, or a segment cut out. The effect 

 of this is to throw an oblique beam of light in one 

 direction across or down the diatom, according to 

 arrangement, and thus to bring into greater promi- 

 nence shadows of the fine striae otherwise almost 

 invisible, owing to their exceeding minuteness. To 

 do this properly requires considerable practice and 

 experience, as may be readily understood when we 

 remember that the "markings" in Amphipleura 

 pelhicida exceed 90,000 to the inch. 



We have alluded to the necessity of making sure 

 beforehand that the mirror and tail-rod are truly in 

 line with the optic axis of the microscope. If this 

 be not the case, the image of the lamp flame will 

 appear to shift in position as the tube of the micro- 

 scope is racked up and down. It is necessary, in 

 addition, that the condenser itself shall be truly 

 centered with the objective. Microscopes fitted with 

 a simple sub-stage ring are only approximately cen- 

 tered, but the mounting of the condenser is generally 

 slightly elliptical, and by rotating the condenser an 

 approximately central position can generally be 

 obtained. The value of a proper centering sub-stage 

 is now evident, and the simplest way of centering 

 the condenser is to make a minute ink-spot in the 

 centre of the top lens, and centre accordingly, after- 

 wards wiping the ink-spot off". Some few condensers 

 were formerly fitted with a small brass perforated cap for 



this purpose. Another method is to focus the image 

 of the iris diaphragm when practically closed, and 

 centre accordingly ; but a little practice enables the 

 microscopist to centre his condenser without any of 

 these aids. 



It may be advisable here to mention that almost all 

 objectives differ in the centering of their mounts, and 

 therefore with high-power work the centering of the 

 condenser will vary for each objective. A rotating 

 nose-piece also disturbs the centering of the objectives, 

 and consequently of the condenser, as no nose-piece is 

 constructed with sufficient accuracy to ensure absolute 

 truth in this respect. 



We think we have now made plain the principle 

 which underlies the use of the condenser, and which 

 amounts really to this — that the light and image rays 

 must coincide. If the condenser be adjusted so that 

 its focus be beyond or belotv the object on the stage, 

 the rays of light will not coincide with the focus of 

 the objective, but will cross, and the result will be an 

 imperfect image. 



There is one other point that may be mentioned in 

 connection with the focussing of the condenser. So 

 far, we have purposely assumed that the source of 

 light has been that of a lamp, which gives practically 

 parallel light. In " Carpenter on the Microscope " 

 it is rightly pointed out that ordinary daylight, owing 

 to its diffusion, does not give parallel light, though a 

 window acts to some extent as a limiting diaphragm. 

 The focus of the condenser differs somewhat, there- 

 fore, for lamplight and for daylight. 



We have already alluded to the use of oblique 

 illumination by means of suitable spots placed beneath 

 the condenser in a carrier adapted for that purpose. 

 There is another form of illumination generally spoken 

 of as "dark-ground illumination." In this case a 

 small central spot, supported on arms, occupies a 

 similar position, and by cutting off those central rays of 

 light which would otherwise enter the objective gives 

 an absolutely dark field in the microscope. When a 

 suitable object is interposed on the stage, however, 

 the' annular rays of light that would otherwise escape 

 the objective are intercepted by the object, and thus 

 diffracted into view. The result is often singularly 

 beautiful — diatoms, foraminifera, and transparent 

 zoophytes being exhibited shining upon an otherwise 

 dark and contrasting background. To obtain the best 

 results it is necessary to bear in mind that the diameter 

 of the stop must be proportioned to the aperture of 

 the objective. Thus a low-powered, and presumably 

 low-apertured, lens will require a small stop, whilst 

 a high-powered and high-apertured lens will require 

 a much larger stop. It would not be of much advan- 

 tage if we were to give means of ascertaining the size 

 of stop required; if necessary, an experimental stop can 

 readily be constructed out of blackened cardboard. 

 The condenser will also require a certain amount of , 

 adjustment ; and a Davis shutter, which is a small 

 iris diaphragm fitted above the objective, by its facility 

 for reducing the aperture of the objective, enables 

 one to obtain perfect background illumination. The 

 spot lens and parabolic illuminator are now but little 

 used. The value of this means of illumination for 

 really critical work we need not discuss here. 



Before leaving the subject of oblique illumination 

 we may just mention that the mere slight tilting of 

 the mirror will often greatly increase the resolution 

 of difficult objects. 



( 7 be continued.) 



Microscopy. — Other articles on subjects con- 

 nected with Microscopy will be found in this number 

 at pages 325 and 337. 



