The Influence of the Cover-glass 65 



power. The optical index, therefore, of an objective which, 

 with an eyepiece magnifying 10 diameters, will yield all 

 that it is possible for a normal eye to appreciate, will 

 be 26"0. In practice it is found possible to employ eye- 

 pieces giving higher magnifications than those mentioned 

 in Mr. Nelson's rule, and these would of course enlarge the 

 image. 



Although large apertures are the pride of those whose ulti- 

 mate ambition in matters microscopical seems to be bounded 

 by the endeavour to resolve the markings upon diatomaceous 

 frustules, it is doubtful whether for the ordinary amateur 

 there is a real necessity for the extremely large apertures. 

 Lenses having such, require great skill and care in manu- 

 facturing and adjusting, and are consequently expensive, 

 and if the ordinary work of an amateur is proposed to be 

 conducted, and not original scientific research, objectives of 

 medium aperture will usually be found to meet his require- 

 ments thoroughly. 



THE INFLUENCE OF THE COVER-GLASS. 



As a rule, objectives are corrected for a specified thickness- 

 of cover-glass, which is placed over the object to protect it. 

 These cover-glasses, however, vary considerably in thickness, 

 and consequently by refraction disturb the corrections of 

 the objectives. An objective which gives crisp definition 

 when an object that has no cover-glass to it is being viewed, 

 will not define so clearly if a thin one be applied, and the' 

 greater the thickness of the cover-glass the more will the 

 image be deteriorated. 



In other words, spherical aberration in the sense of 

 under-correction is introduced when the cover-glass is- 

 thinner, and in the sense of over-correction when the 

 cover-glass is thicker, than that for which the lens was- 

 adjusted. This spherical aberration arises from the refrac- 

 tion of the rays in the plane surfaces of the cover-glass and 

 objective front respectively ; it is negative for the cover- 



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