SC/KXC/C-GOSS//'. 



61 



MICROSCOPY FOH BEGINNERS 



r.\ i'. SmiiLixoTON Si u.i -. F.R.M.8. 

 (Continued from pagi 29.) 



When the sfcudenl is making accurate micro- 

 metric measurements, ii should be remembered 

 1 1 1, -it the divisions on the stage-micrometer are not 

 uniform, and thai the mean value should be 

 accordingly taken. Further, the stage-micro- 

 meter should be used in every sel of measure- 

 menl s; the usually recommended plan of making a 

 record with differenl objectives and eyepieces, 

 once for all, for comparison, being manifestly un- 

 trustworthy. Errors due to diffraction in tin- real 

 edge- of objects mainly affect high powers, and 

 we may contenl ourselves with simply mentioning 

 their existence. 



The measurement of the actual magnification 

 due to an objective alone i- best made by project- 

 ing the image of the stage-micrometer, without 

 eyepiece, on a cardboard screen five feet away, by 

 marking and then measuring t lie distances between 

 the lines, then dividing by 6, which will give the 

 actual magnification of the objective atthe normal 

 visual distance of ten inches. At the five-feet 

 distance the error in the measurement will be 

 Inappreciable. The additional magnification due 

 to the eyepiece can lie measured by projecting the 

 image of the stagej-micrometer through both objec- 

 tive and eyepiece with the 10-ineh tvie, and dividing 

 by the known magnification of the objective. This 

 tube-adjustment must be most carefully made. 

 The distance of the projected image in this latter 

 case, measured from the eye-lens, must also be 

 the normal one, i.e. 10 inches, either direct or with 

 tin: camera lucida. If the objective should be an 

 inch of a proved magnification of ten, and the 

 combined magnification at ten inches from the 

 eyepiece under the above conditions be (50, the 

 magnification due to the eyepiece will be t>. 

 What we have so frequently said about variations 

 in tube-length must lie strictly borne in mind in 

 making such measurements or calculations, as 

 every variation in tube-length leads to a corre- 

 sponding variation in the magnification due to the 

 objective alone. The theory of microscopic vision 

 should be remembered — i.e., that the image of the 

 object. as magnified by the objective, is magnified 

 again by the eyepiece, but that the magnification 

 of the latter is constant, whilst that of the former 

 varies according to whether it is focussed nearer 

 to or farther from the object, thus altering tin- 

 position of the image referred to above ; in other 

 words. and speaking practically, it varies with the 

 tube-length. It must also be borne in mind that 

 the foregoing data are applicable, strictly speak- 

 ing, only to those of normal sight, and who in 

 consequence form their image at the calculated 

 and ordinary distance of ten inches from the eye. 

 All others having abnormal vision must use lenses 

 or spectacles to correct the difference. 



The eyepieces, like the objectives, should be 

 kept clean and free from dust. Tin- dust can be 

 readily localised by reducing the light by means of 

 the iris-diaphragm or otherwise, and then rotating 

 the eyepiece. The objective- should not give 

 much trouble in this wav. and it will often be 



found that what was apparently dust on the back 

 lens of the objective was actually dust on the 

 cover-glass of tin- object or on the top lens of tin 

 condenser. This can in- Localised in like mat 

 by moving the slide or rotating the condenser. 



I be use of i he polariser and analyser wouL 

 more space than we can give to ii here. We maj 

 simply say that with "crossed nicols" any object 

 showing crystalline structure or possessing double 

 refraction will show more or less brilliant colours 

 i >n a black ground, and crystals, mineral sections, 

 fibres, odontophores, &c, are very advantageously 

 exhibited by this means. With parallel nicols the 

 background will be bright . but I he colours will —till 

 be in evidence, though the tints will vary, !'•■ 

 rotating the analyser or polariser, most beautiful 

 effects can be produced, and with weakly refracl 



objects as gorgeon- effect.- Call be produced by 



inserting a slip of selenite, for crystallographic 



or penological work an analyser and polariser are 

 necessities, and elaborate microscopes fitted with 

 many other adjuncts are sold for this purpose. 

 Unfortunately the various books on the microscope 

 deal very lightlywith the subject, and those who 

 require information on this and other matter- con- 

 nected with the polarising microscope anil it- 

 are referred to a paper by the present writer in the 

 current issue of tin- ••Annual of Microscopy" 

 1900. 



In concluding these elementary hints on the 

 actual management of the microscope, we would 

 strongly advise our readers to obtain a few ■ 

 slides," and diligently to practise themselves in 

 their examination. For low powers there i.- nothing 

 more suitable than the fine hairs on the tip of the 

 proboscis of the blow-fly. These should come out 

 quite sharp, black, and finely pointed. For medium 

 powers nothing is more suitable than the old- 

 fashioned "podura scale" (Lepidocyrtis curvicollis) 

 mounted dry, and if the beginner can get the 

 "hairs" on these sharp and well marked, having 

 the appearance of minute exclamation stops, and 

 not running in the least into each other — we say 

 nothing of interior " marks " — he has learnt much in 

 the management of his microscope, and in obtain- 

 a " critical image." Unfortunately the scales them- 

 selves vary very much, and it is not always easy to 

 pick out a coarse enough scale that will be within 

 the beginner's or even the objective's powers. 

 In obtaining such a critical image the worker will 

 bear in mind not only what we have already said 

 in our earlier papers on adjustment of the light by 

 means of the condenser and otherwise, but our 

 warning against an excessive shutting down of the 

 cone of illumination by means of the iri— dia- 

 phragm. This latter, whilst apparently at first 

 sight increasing the contrast, really breaks down 

 the definition of the objective, and leads to woolli- 

 ness of outline in each individual mark on the scale. 

 or even to a ring of refracted light round each. 

 For high power-, especially immersion lenses, we 

 prefer a slide of bacteria, say a slide of Bacillus 

 1 a hen- n las/, a. showing the characteristic beaded 

 appearance. The diatom Pleurosigma evngulatum 



is also a Useful test object. We mention the- 



helpful aids in mastering the use of the micro- 

 scope only. The testing of objectives requires 

 practice, skill, and experience, though we pro] - 

 at a not too distant date, to give the reader- ot 

 this journal some notes on the matter, and t.> 

 illustrate them with suitable photo-micrographs. 



( To be conduit 



