SCfRNCn-(;OSSIP. 



59 



MiCR()SCoi'\' lou ui:oinm:i<s. 



By F. Smi-LiNtiTON ScAi.i-.s, !■'. K..M.S. 



In few Ihinys is a litllc initial udviec and hcli) 

 more- valual>le, than wIilti one is tirst inking; ii|> ll<c 

 use uf an instrumonl liku the niicri>scop<:. We arc 

 cunstanlly ntvvtiiig lliose wlm arc desirous of 

 inirchasiiig a niicri)si:ii|ie, and then learning liou to 

 use it ; but who feel themselves l>ewiUlere<l at the 

 very outset l>y the multiplicity of staixl'^ and 

 accessories advertised hy the various Knglish and 

 Continental makers. There are several escellent 

 manuals lo he obtained on the suhject, but the keen 

 competition of the present day makes it impossible 

 for any book to keep up to date with the latest 

 stands, etc. This especially applies t<i tho.se less 

 expensive and most recent stands, with which we 

 shall mainly concern ourselves. .\l the outset, as 

 the title of these papers plainly states, we shall 

 disclaim any idea ol tcichiiij; more experienced 

 workers. Our aim is simply to indicate to the 

 beginner, who, with uuich enthusiasm, little or no 

 knowledge, and perhaps not too well lined a purse, 

 propixses to take up study with the microscope. We 

 shall indicate how he may to most advantage spend 

 his money on the necessary outfit ; what in fact are 

 the es.sential requirements that he must look for in 

 his instrument, whoever is the maker, l-'urther, we 

 shall explain uhat accessory apparatus will be 

 required in addition. We .shall then give a few- 

 practical hints on elementary manajjement, on 

 mounting, etc. The cost of everything will be kept 

 as low as possible, hut wc shall not concern our.selves 

 with an outfit suitable only for whiling away a 

 pleasant hour or two, for "looking at pictures" in 

 fact. Instead we hope to so advise the beginner, 

 that afterwards, when he has gained experience for 

 himself, he may feel that what he has bought is 

 capable of showing him not only the infinitely little 

 in Nature, but of enabling, if he .so desires, lo do real 

 and serious wi>rk in medicine, in zoology, in botany, 

 or in any other of the countle.ss ways in which the 

 modern microscope has become a necessilv. 



Let us start by enumerating the essential points 

 that we shall require in our modern luicroscope. 



There are practically two classes of stands- that 

 with the pillar ami horse-shoe, or equivalent 

 form of foot, and that with the later tripod. The 

 advantage of the furmcr is that it is generally a little 

 cheaper, and the sub-stage apparatus is more con- 

 veniently manipulated when the microscope is used in 

 an upright position, as often happens to zoological 

 and medical workers. In any other than the upright 

 position, it is much less steady than the tripod forui. 

 Kor horizontal work, especially for micro-photography, 

 it is quite top-heavy. The importance of this point 

 can only be fully appreciated and veriticd by the 

 practical worker. The result is that the tripod foot 

 is daily coining more into favour, and we unhesi- 

 tatingly advise it. The stand .should of course be 

 capable of inclination. 



There must be a thoroughly reliable fine adjustment 

 for focussing high powers. Here we would most 

 strongly warn the intending purchaser against 

 the kind of fine adjustment that n\oves the 

 nosepiece at the foot of the tulie. Most 

 of the stands seen in provincial, and even in 

 many ]>i>ndon shops, have this kind of adjustment. 

 Inn it is to be avoided, as sooner or later it will give 



its ow ner trouble. U'e will allude to more .salisfoclory 

 forms, as we mention individual stand.s. \ 



The coarse adjustment may l>e by either rack and 

 pinion, the Iwst makers fit now a ilia^oiia/ rack 

 that gives most soft ami steady motion, without back- 

 lash, or it m.iy l>e an ordinary sliding adjustment 

 only. <Jf course the former is preferable for anything 

 but rough work. The reader will fiii<l the sliding 

 adjustment perfectly satisfactory as far as it goes, and it 

 often saves about 25 per cent, on the cost of the stand. 



The screw for the objectives at the lx>ttoiii of the 

 tube should be what is called the " Society" size. In 

 other worils the size adopted and standardised by the 

 Koyal Microscopical Society, which provides the 

 makers with the necessary templates. Nearly all 

 good Knglish objectives are now made to this size, 

 and it has been adopted by best Continental makers. 



The eyepieces unfortunately are less uniform in 

 size, and, until recently, every maker had a size of his 

 own. The Koyal Microscopiail Society recommends 

 two standard sizes, the English size i '35 inch, and the 

 Continental size -92 (about | J) inch. Most students' 

 stands are made for the latter or some approximate 

 size, which we regret, as the larger eyepiece has 

 several advantages. The mere diameter of the 

 eyepiece does not, however, affect the magnification. 



The Continental length of the tube is about 6 inches 

 (say 160 millimclresl, and the Knglish length of tube 

 i.s 10 inches (say 250 millimetres) : this la.st being 

 taken as the normal visual distance. We have no 

 space for an explanation of the optical theory of the 

 compound microscope, but we may here state that the 

 aerial image of the object magnified bv the objective 

 is itself magnified by the eyepiece or ocular. The 

 magnifying power of the individual ocular remains 

 constant — that of the aerial image given by the indi- 

 vidual objective depends upon the distance between 

 objective and ocular. Therefore an objective and 

 ocular used in a tube 6 inches long will give only VV 

 of the magnifying power of the same objective and 

 ocular in a to-inch tube. We will allude to the 

 practical bearings of this when we deal with objec- 

 tives. It .suffices to say here, that objectives made, 

 or, as it is technically called, "corrected' for the one 

 tube length, cannot be used for the highest class of 

 critical work with the other tube length ; though low 

 powers, with low eyepieces bear the alteration fairly 

 well. It is in consequence a gieat advantage to have 

 a draw-/ nil,' that lengthens from 6 to 10 inches, so 

 that oljjectives correcteti for either length may lie used 

 on the .same microscope. This draw-tube has also 

 other uses that will be alluded to later. It is a sine 

 i/iia lion, when selecting a new instrument. 



The stage should lie large and rigid. The aperture 

 should be oi sufiicient size lo permit of the finger 

 being in.serted under the slide to enable one to " feel 

 one's distance," when focussing high powers. One 

 or two makers cut out the front of the aperture so as 

 to make a horse-shoe opening. For use without the 

 conden.ser this has some advantages. A mechanical 

 stage is a convenience, but is not necessarv for our 

 purpose and is moreover expensive. It can often be 

 added later. The mirror should have two faces, a 

 plane and a concave one. The plane face should be 

 " parallel worked" as it gives a truer reflection of the 

 light. The whole should swing in a gymbal and 

 slide up or down on a tail rod. We piefer that the 

 latter should be circular and itself swing on a pivot, 

 just beneath the stage. 



(To I'f conlrnueif.) 



