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SCIENCE-GOSSIP 



MICROSCOPY FOR BEGINNERS. 

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

 (Conthined from page I5y.) 

 High power objectives of high class are gen- 

 erally provided with a rotating collar that 

 enables the observer to adjust the distance between 

 the combinations of the objectives so as to correct the 

 lens for variations of cover glass, but this requires 

 more practice than most students have had, and a 

 very general way is to shorten or lengthen the tube, 

 which has the same effect. Thus, supposing the 

 objective is corrected for a cover glass .008 inch thick, 

 it would be necessary to shorten the tube, i.e., de- 

 crease the distance between eyepiece and objective 

 if a thicker glass were used, or to lengthen it for a 



HuYGHENIAN EvEPIECE. 



Kellner Eyepiece. 



thinner glass. By so doing we should, as already 

 explained, also alter the magnification. With a 

 correction collar we should close, the lenses of the 

 objective for a thicker cover, and open them for a 

 thinner one. 



It would be a great convenience if makers would 

 adopt one uniform thickness of cover glass to which 

 all objectives could be corrected. One well-known 

 maker corrects for a cover glass '005 in. thick, 

 whilst another corrects for a cover glass of 'craS in. 

 It may also be necessary to warn beginners that the 

 magnifications of objectives are loo often greatly in 

 excess of what they profess to be, it being 

 easier to make a higher power objective than 

 a lower one of the same aperture. We think 

 . the present custom of sending out most 

 student's objectives corrected foi^ a 6 in. tube 

 is a mistake, as the 10 in. is still the 

 standard English tube length. Under any 

 circumstances it would be a great advantage 

 if makers would mark on their objectives, as 

 one or two firms already do, not only the 

 N.A. and the actual focal length of the 

 objective, but the tube length for which it is corrected. 



Let us now deal with the necessary eyepieces, and 

 on this matter fewer words will suffice than we have 

 thought it advisable to give to the subject of 

 objectives. The "compensating eyepieces" are 

 specially constructed to correct certain outstanding 

 errors in the apochromatic objectives, and are 

 necessary therefore only for - that purpose. The 

 eyepieces commonly used with achromatic objectives 

 are the " Iluyghenian " or "negative," and the 

 "Keihier" eyepieces. The Huyghenian eyepiece is 

 composed of two plano-convex lenses with a limiting 



diaphragm in the principal focus of the eye-lens. 

 The diaphragm cuts off the marginal rays, which are 

 useless owing to their too great spherical aberration, 

 but this very aberration helps to correct the 

 aberrations of the objective and so flatten the field. 

 The Kellner eyepiece has no diaphragm and its field 

 is therefore large, being in fact only limited by 

 the size of the eyepiece tube. It has an achromatic 

 meniscus for the ocular and a double convex field lens 

 lying in its focus. Thougli the field is large, the 

 definition is not as good as that of the Huyghenian 

 ocular, and any dust is very apparent. It is used 

 mainly for micrometry, but a micrometer is very 

 frequently inserted in the focal plane of the 

 Huyghenian lens. With projection eyepieces we need 

 not concern ourselves. 



Eyepieces are made of different powers, ranging 

 from 4 magnifications up to 15, and even higher. 

 The 4 eyepiece often is unsatisfactory with the short 

 tube length ; the most generally useful is the 6 eye- 

 piece. After this, one of, say, 10 magnifications. 

 Above this it is useless to go. We are quite aware 

 that all makers sell eyepieces of much higher powers, 

 as mentioned above, but we have never seen an 

 achromatic objective that would satisfactorily and 

 critically bear eyepiering above ten times with a 

 10 in. tube length. ; It is only the apochromatics that 

 will bear high eyepiecing, and even in their case the 

 loss of light and depreciation of the image is notice- 

 able. But until the apochromatics were invented the 

 only way to get hjgh magnification was to use high 

 power objectives, such as ^jV and ^V in., which we so 

 seldom hear of nowadays. 



Before leaving the subject of eyepieces, we would 

 again reiterate what has been so frequently urged 

 upon makers — namely, the manifest disadvantage of 

 marking their eyepieces with any other numeral or 

 letter than the actual number of magnifications 

 given by the eyepiece at the standard 10 in. tube 

 length. It then becomes an easy matter, as already 

 explained (Science-Gossip, Vol. vi., p. 157) to esti- 

 mate the magnification with any eyepiece or objective, 

 and when used in any tube length. , 



We come now to the subject of the sub-stage con- 

 denser, generally called "the condenser," whereas 

 the bull's-eye condenser is spoken of as " the bull's- 

 eye " simply. This is practically of two types, the 

 "Abbe illuminator," and the achromatic condenser. 

 The Abbe illuminator is itself .sold in two types, a 

 double combination with an aperture of 1.2 N.A. and 

 a triple form of 1.4 N.A. Of course, to get the whole 



1.2 N'.A. 1.4 N.A. 



Optical Part of Abbe Illuminator. 



of this aperture, the condenser must be in immersion 

 contact with the under side of the object slide. Both 

 are simple, cheap, and easy to work, and are 

 accordingly largely used by students ; but in both the 

 spherical and chromatic aberrations are enormous, and 

 they are therefore not satisfactory when used for 

 critical work. The actual aplanatic cone is in both 

 cases under .5 N.A. At the same time, the beginner 

 will find it a very useful form of condenser for 

 general work. We prefer the 1.2 N.A. type, and it 

 is also the cheaper of the two. 



{To be conitnucd) 



