3H 



SCIENCE-GOSSIP. 



MICROSCOPY FOR BEGINNERS. 



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



{Contiytued /toih page 280.^ 



We propose now to deal with the elementary 

 management of the microscope and its accessories. 

 The modern microscope has become an instrument of 

 such exquisite precision that the beginner generally 

 finds himself at the very outset in need of explanation 

 and assistance in this respect. A clear understanding 

 of what he is doing is necessary if he is to obtain the 

 best results. In our medical and other laboratories 

 too little attention is often given to these matters, 

 and it follows, therefore, that the student is not really 

 familiar with the use of his instrument, and obtains 

 more or less imperfect results. The general use of 

 the sub-stage condenser has revolutionised micro- 

 scopy, and we would here call attention most strongly 

 to the fact that the proper management of the illu- 

 mination of the object is a matter requiring as much 

 or even greater care and attention than the actual 

 focussing of the objective itself. This last is com- 

 paratively a simple matter, save where corrections 

 have to be made for diverse thicknesses of cover- 

 glasses. 



We will assume that the reader has provided him- 

 self with a microscope fitted up in accordance with the 

 rules and suggestions that have been the subject of 

 the preceding papers of this series ('). He will place 

 his microscope on the table before him, and incline it 

 at a comfortable angle for looking down the tube. 

 Of course there is a certain amount of "work that can 

 only be done with the microscope in a vertical posi- 

 tion ; but when the object will admit of it, it is a great 

 convenience and rest to the muscles of the neck if the 

 microscope be slightly inclined. This is a point that 

 makers of even the less expensive foreign microscopes 

 are beginning to realise, and one by one the English 

 improvements are being adopted on the Continent. 

 It will be found that ordinaiy daylight, though per- 

 fectly suitable for low-power work, is unsuitable for 

 high-power and critital work, and as we are setting 

 up our microscope for critical work we will deal with 

 lamp-light illumination, accordingly. We need 

 scarcely warn the beginner that the use of direct sun- 

 light is absolutely out of the question for visual pur- 

 poses. The lamp will be probably an ordinary 

 half-inch paraffin lamp, with a shade, as already 

 described, but without a reflector, as this last only 

 succeeds in confusing the rays of light. Some 

 workers place the lamp on the immediate left 

 of the instrument in order to obviate any glare 

 into the eyes, but the position immediately oppo- 

 .site and m front has so many advantages in ease 

 ■ of manipulation that we prefer this position ourselves, 

 and interpose a cardboard shade if required. The 

 lamp will therefore be placed in the latter position, 

 •with its wick turned edgeways forward, the light 

 being about 6 to 10 inches or thereabouts from the 

 mirror. It is well to accustom oneself to these ap- 

 proximate distances, as high-power immersion-con- 

 densers are adjusted amongst other things for a definite 

 distance of the light from their back lenses. It will 

 be necessary to carefully observe at the outset that 

 the flame is immediately opposite the centre of the 

 stage, and that the tail-rod and the mirror are ap- 

 pro.ximately truly in line with the optic axis of the 

 microscope. This is important. A slight tilting of 

 the mirror will then be sufticient to fill the tube with 



(i) This series of articles on the Microscope commenced in 

 the July 1899 number of Science-Gossip. 



light. For an objective lower than \ inch or 

 \ inch the condenser will not generally be required, 

 and in this case the concave mirror and the flatsideof 

 the flame can be used. The concave mirror should be 

 adjusted at such a distance from the object on the 

 stage that the rays of light are approximately focussed 

 on the latter. A little reflection will show our readers 

 that when parallel, or nearly parallel, light falls on 

 a concave mirror the light becomes coiivergeiit^ and 

 the focus will depend upon the curve of the mirror 

 itself. If the condenser be retained for these low-power 

 objectives, the upper lens should be removed or the 

 field of illumination becomes inconveniently circum- 

 scribed ; but for the reasons just given the plane 

 mirror must always be ttsed with the co7idenser. We 

 have not infrequently seen workers, otherwise ex- 

 perienced, having noticed the increase of light ob- 

 tained by the use of the concave mirror wifhotit the 

 condenser, using it for the same purpose with the 

 condenser, but forgetting that by so doing they are 

 throwing upon the condenser convergent instead of 

 parallel light, and making it impossible to satisfac- 

 torily focus the condenser itself. 



For objectives above f inch the use of the 

 condenser is advisable, and for higher powers im- 

 perative, if the full value of the objective is to be 

 realised. In Continental microscopes, until recently, 

 a sub-stage condenser was the exception rather than 

 the rule. In many of our English laboratories its 

 use, therefore, is even yet scarcely understood, 

 because of the curious prejudice in favour of the Con- 

 tinental stand ; but though the Continental makers 

 turn out excellent objectives, they have only latterly 

 begun to realise that an objective of high aperture 

 has really no advantage over one of low aperture 

 unless used with a suitable condenser. 



Having so arranged lamp and mirror that the 

 object is properly illuminated, it will be necessary 

 then to focus the objective upon the slide, and, having 

 done that, to adjust the condenser up or down, until 

 the image of the edge of the lamp flame appears dis- 

 tinctly in the centre of the field. The light is thus 

 correctly adjusted, its rays being brought to a focus 

 upon the object, and so entering the objective un- 

 disturbed. To a beginner, the partially-illuminated 

 field seems strange ; but he must remember that in a 

 critical image it is necessary to get the maximum 

 perfection of definition in that portion of the object 

 immediately under examination, and the rest may be 

 ignored. If, however, a larger field be required, the 

 flame, having been focussed, can then be turned 

 broadside on, without altering its other positions. 

 For ordinary work this is usual. 



In earlier days it was customary, after focussing 

 the light, to rack the condenser either up or down 

 until the field was full)' illuminated, and we have 

 heard that the Quekett Club, which has done so much 

 for amateur microscopists, held two vehement rival 

 schools, one of which maintained that the condenser 

 should be racked up and the other that it should be 

 racked down. All these matters have now been 

 settled by the realization of the real principle, and 

 even more, perhaps, by the improvement in ob- 

 jectives, by which, under such illumination as now 

 obtains, hidden structure has been brought to light 

 that was before not even suspected. It may be well 

 to warn beginners against racking the condenser so 

 high as to drive the slide up against a high-power 

 objective. 



( To be continued. ) 



