SC/ENCE-GOSS/r. 



I 3/ 



\11( K')M Dl'V IDK IJKl.lNNKKS. 



Hv F. SllILLlSHTON SCALBS. F.R.M.S. 



( Cimliniied from fia,i;e 12$.J 

 li «ill havf been noticed thai the stands we 

 nave mentidned are i|Uote(l with an inch and a 

 \ inch or '„ inch objective. It generally pays to make 

 the whoL' purchase at once, as the makers charge a 

 little less in such cases than if the items are lioiight 

 separately. 



It may be as well to state that the terms i inch, | inch, 

 etc., <lo not represent the distance between the under 

 side of the lens and the object. The tendency of the 

 d.iv is to increase the aperture, bringing the lenses 

 closer and closer to the object, and the designations 

 referred to may be taken practically as representing 

 approximate magnifying power. Taking the inch as 

 a standard, we may say that «illi>>ul any eyepiece it 

 gives in a to inch lube a magnifying power of 

 approximately to diameters. .\ | inch gives 40 

 diameters, a' \ inch 60 diameters, a 2 inch 5 

 diameters, and so on. To get the actual magnifica- 

 tion one must know the magnifying power also of the 

 eyepiece, and the length of tube. If the eyepiece 

 magnifies 6 times, then an inch objective with this 

 eyepiece and a 10 inch tube, will give a total 

 magnitii-alion of 60 diameters •. a ', inch will magnify 

 240 diameters, and so on. With a 6 inch tube the 

 magnification will be only ,"„' of the foregoing, and 

 this must be borne in mind. Opticians have a 

 tendencv, however, to give rather larger magnifi- 

 cations than those we have mentioned, owing to the 

 difficultv of making low power objectives of as wide 

 aperture as the taste of the day demands, and this 

 should be noted when purchasing. The higher the 

 aperture the better should be the definition, but the 

 higher also is the price, and there is the additional 

 disatlvantage to many workers, that anything out of 

 the exact plane in focus is practically invisible. Most 

 workers use objectives of bith high and low aperture 

 and each have their advantages. 



Theoretically olijectives should have .26 X.A. for 

 each too diameters of combined magnification, but 

 in practice very much less than this suffices, ilessrs. 

 Beck for instance make most of their objectives of 

 comparatively low aperture, not only because they 

 are less expensive, but because they give more of what is 

 known as "penetration," or the power of seeing 

 more than lies in the one actual plane. Moreover a 

 high aperture reduces the working distance. The 

 tendencv of the day, however, is to insist on high 

 apertures, and each maker tries to outdo his 

 competitors in this respect. Our advice to beginners 

 is to be content with moderate apertures, at any rate 

 at first, but, above all, to get objectives made by 

 lirst-rale houses only. If of other than English 

 make see that the screw is of the " society " g.auge. 



\Vc had perhaps better say a word or two here on 

 the subject of Numerical Aperture, or, as it is 

 generally written, N.A. P'or a full discussion of 

 The Abbe theor)- of microscopic vision and the use 

 of immersion lenses we must refer the reader 

 to the standard text-books, such as "Carpenter 

 on the Microscope." If the reader will, how- 

 ever, bear in mind the well-known law of 

 physical optics, /.<■., that rays of light passing from a 

 den.ser medium into a rarer one are refracted /;wh the 

 perpendicular and viie rvv.fii, he will iimierstand that 

 terms representing " angular aperture," 'h the cone of 

 light embraced by the objective in air. could not be 

 used to represent the aperture of lenses that had a 

 drop of liquid of high refractive index, such as water 

 or oil. between them and the covei^lass. In fact a 



water inniiersion of 97" and an oil immersion of 82'' 

 are each eipiivalent to an air lens of iSo". Accord- 

 ingly a formula has l)cen universiUly adopted by which 

 I N..\. represents an angle of iSo" in air, which is 

 rather higher than any air lens can grasp in pr.ictice, 

 and in estimating the numerical aperture of objectives 

 Cognizance is taken not only of the angle of a|)erlure 

 of the objective but also of the refractive index of the 

 medium in which it is designed to work, whether air, 

 oil, or water. The formula is, nutltiply the sine of 

 half the angle of aperture by the refractive index of 

 the meiliuni. Therefore, according to this fornmla, 

 I N.A. will represent a corresponding angle of 97" in 

 water and S2*'' in oil. The maximum theoretical 

 angle for water is 1.33 N..V. I'or oil of the same 

 refractive index as a crown gla.ss cover it is 1.52. 

 With special immersion medium and glass, Messrs. 

 Zeiss have made a lens with an aperture of no less 

 than 1.6 N.A., but it is of course proportionately 

 costly and can only be used for special work. 



Hefore leaving the subject of objectives we must 

 touch brietly on what are called their "corrections." 

 .\n objective at the present time is a very complicated 

 art'air. It is composed of two or m()re lenses of 

 certain carefully calculated .and definite curves, set 

 at e<|ually definite distances. .Most of the individual 

 single lenses that go to make up the combination are 

 made of two dilTerent gl.asses, such .is crown glass anil 

 flint glass, joined together, in which the curves of the 

 one correct certain aberrations in the other. These 

 aberrations are of two .sorts, "spherical aberration," 

 or the non-coincidence of the margin.il and central 

 light rays in the objective, causing a curved field, and 

 "chromatic aberration," or the non-coincidence of 

 the colours that the lens, acting as the prism of a 

 spectroscope acts, has fanned out and failed accurately 

 to re-focus and combine again. This lea<ls to 

 outstanding colour, and though no achroniatics .are 

 ([uite free from this fault it should be only perceptible 

 on critical tests. .V portion of the outstanding colour 

 is got rid of by means of the diaphragm, which cuts 

 off the ill-corrected marginal rays. 



Now, we need scarcely s,ay that an objective that 

 has been corrected for air cannot satisfactorily be used 

 as an immersion lens, and rvVi- ;;-r.ui, but it may be 

 necessarv to point out that an objective is likewise 

 corrected for a certain length of tid)e and can- 

 not be used critically for any other length 

 of tube. This, whilst showing the advantage of 

 having a draw-tube that will enable either short or 

 long tube objectives to be used at will, shows also the 

 limitations of variati.in of magnification bv the simple 

 device of drawing out or closing up ihe draw-tube. 

 There seems to be an increasing tendency amongst 

 ICnglish opticians of repute to correct their objectives 

 for the short or continental tube length. This we 

 regret for many reasons, one of which is that an 

 objective corrected for the 10 inch tube performs better 

 on a 6 inch tube, if necessary, than in the reverse case. 

 .\nd we especially protest against the practice of 

 certain makers of quoting their objectives as giving 

 certain magnificationswith a loinch tube, but refraining 

 from adding the necessary qualification, that the said 

 objectives are in reality corrected fur the 6 inch tube 

 onlv. Kurther, objectives are corrected for a certain 

 uniform thickness of covetglass, though there is, un- 

 fortunately, no uniformity amongst opticians in this 

 respect, ami any alteration in either lube-length or 

 coverglass thickness upsets these corrections. Of 

 course, this is only noticeable with what is called a 

 "critical image." which the eye needs educating to 

 perceive. 



( To he lOiiliiiiied), 



