ADVANTAGES OF APOCHEOMATIC OBJECT-GLASSES 33 



inalienable from an objective constructed of ordinary crown and 

 flint glass, can be, and have been, eliminated, and the secondary 

 .spectrum annulled ; it is removed and reduced to a residue of 

 chromatism of a tertiary character, while the chromatic difference 

 of spherical aberration can be eliminated or completely corrected 

 for two different colours of the spectrum at once, and therefore 

 practically for all. 



In the lenses formed of the crown and flint glass as used prior 

 to the new German glass, we were .provided with what (in com- 

 parison with non-achromatised lenses) were called ' achromatic ; ' 

 but in the new system of lenses, which may be ' dry ' or ' homogeneous,' 

 we have so great a freedom from colour defect as to admit of their 

 being designated apochromatic lenses (a=privative ; gjHtyifi&seoloiir ; 

 a7ro=from, away from ; xpc!>/ia=colour). 



The practical advantages obtained by this system of object-glass 

 construction are so great as in delicate researches to be invaluable 

 provided always that the work in all its details is of the most perfect 

 kind. The accidental juxtaposition of lenses of the required curves, 

 and, relatively, even the careful selection of lenses not homogeneously 

 related to each other by a unity of purpose and work on the part of 

 the practical optician, cannot yield perfect results. ' Division of 

 labour ' is not compatible with perfect results in the making and 

 building up of an apochromatic lens; and therefore, in their best 

 form, these objectives must apparently command a high price. But, 

 given such an object-glass which is the production of a thoroughly 

 competent practical optician and its advantages, theoretical and 

 practical, are great. 



1 . The aperture of the objective can be utilised to its full extent. 

 In the best of the older object-glasses at least one-tenth of the 

 available aperture was useless ; the inalienable defect in the con- 

 vergence of the rays prevented a proper combined action of the 

 outermost zone and the central parts of the aperture, and therefore 

 by those objectives it has never been possible to realise the amount l 

 of resolving power indicated by theory with a given aperture. But 

 in a well -constructed apochromatic objective the secondary spec- 

 trum being removed, and the spherical aberration being uniformly 

 corrected for different parts of the spectrum there is a practically 

 perfect focal concentration of the rays in the image. 



2. Increase of magnifying power by means of specially constructed 

 eye-pieces is also a most important feature of objectives of this class. 

 The result of this is that great magnifying power can be obtained 

 by objectives of relatively large focal lengths. We have always 

 maintained the utility of high eye-piecing under proper conditions, 

 and with suitable apertures and fine corrections in the objective ; 

 the physical brightness, w r e learn from Abbe, in every case depends 

 only upon the aperture and the total magnifying power ; and it is 

 of no moment in what way the latter is produced by means of focal 

 length of the objective, length of tube, and focal length of eye-piece. 



1 Excepting when resolution is effected by light of extreme obliquity. If the 

 outermost zone of the objective is corrected alone, and that only be employed, at that 

 limit equally good resolution may be accomplished. 



D 



