[ 91 ] 



apocbroniatic ©bjectivcB for tbe flDicroecope^ 



MR. J. G. Wright publishes the following paper in iho. Journal 

 of the Btrmmgha7?i Natural History and Philosophical 

 Society (Vol. I., 1894, pp. 57 — 61) : — 



" A great advance has been made during late years in optical 

 science as applied to microscopical work, through the laborious 

 and exhaustive investigations of the subject by Prof. Abbe and 

 Dr. Schott of Jena. Those gentlemen, after years of patient labour 

 and study, have discovered new vitreous compounds, by the use of 

 which, and aided by the celebrated optical workshops of HerrCarl 

 Zeiss, they have constructed lenses capable of forming images 

 much more perfect than any hitherto seen, and indeed have lifted 

 the art of the construction of microscope 'objectives' into an 

 almost ideal position. 



The nature and advantage of these improvements will be 

 understood from the following considerations and explanations. 

 The ideal desideratum in a lens or a combination of lenses is to 

 produce an image of perfect definition, free from all colour inter- 

 ferences. There are, however, some serious difficulties in the way 

 of realising it. When rays of light pass through a lens, those 

 passing through the axial zone are focussed at a point further from 

 the lens than those passing through the peripheral zone, while those 

 passing intermediate zones focus themselves at points intermediate 

 between those two. Hence the image formed by the lens is 

 imperfect in any one of these focal points. This difficulty is 

 known as ' spherical aberration.' 



When a beam of light passes through a lens, the various 

 coloured rays are refracted at different angles. Thus, the violet 

 rays are focussed at a point nearer to the lens than the red rays, 

 and the intermediate colours at varying points between those two, 

 so that the image viewed at any one of these foci is more or less 

 imperfect and blurred. This difficulty is known as ' chromatic 

 aberration.' Both these difficulties are partially corrected by a 

 so-called ' achromatic ' arrangement of lenses, in which a bi-convex 

 lens of crown glass is closely attached to a plano-concave lens of 

 flint glass, the latter possessing a lower refractive index and a less 



