28 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS 



system ' allows of a greater working distance between the objective 

 and the object than is otherwise attainable with the same extent of 

 aperture ; and this is a great advantage in manipulation. Further, 

 the observer is rendered less dependent upon the exactness in the 

 correction for the thickness of the covering glass, which is needed 

 where objectives of large aperture are used ' dry ; ' for as the 

 amount of ' negative aberration ' is far smaller when the rays which 

 emerge from the covering glass pass into water than when they pass 

 into air, variations in its thickness produce a much less disturbing 

 effect. And it is found practically that ' immersion ' objectives 

 can be constructed with magnifying powers sufficiently high, and 

 apertures sufficiently large, for the majority of the ordinary pur- 

 poses of scientific investigation, without any necessity for cover-ad- 

 justment ; being originally adapted to give the best results with a 

 covering glass of suitable thinness, and small departures from this 

 in either direction occasioning comparatively little deterioration in 

 their performance. But beyond all these reasons for the superiority 

 of the ' immersion system ' is, as will be presently seen, the fact that 

 it admits into the lens a larger number of ' diffraction spectra ' than 

 can be possibly admitted by a lens working in air ; and upon this 

 depends the perfect presentation of the image. 



The immersion system has still more recently been advanced upon 

 by the application of a principle which lies at the root of the optical 

 interpretation of the images which modern lenses present, and 

 which has greatly increased the value of the microscope as a scientific 

 instrument. It is an improvement that primarily depends upon a 

 correct theoretical understanding of the principles of the construction 

 of microscopical lenses, and the interpretation of the manner in 

 which the image is realised by the observer. The late Mr. Tolles 

 was the first to adopt this system, as we point out subsequently ; 

 but it is to Professor Abbe we are indebted for its practical appli- 

 cation, through whom it is now known as the homogeneous system. 

 The word ' homogeneous ' was, however, first applied to microscope 

 lenses by Tolles (1871), as may be seen in the following passage. . . . 

 ' two hemispherical lenses balsam-cemented, with a diatom or other 

 small object at the centre, together constituting a nearly homo- 

 geneous transparent globe' (M. M. J., vol. vi. p. 214). 'The idea 

 of realising the various advantages of such ' a system by constructing 

 a certain class of homogeneous objectives had, Professor Abbe 

 says, l 'for sometime presented itself to his mind.' 'The matter 

 assumed, however, subsequently, a different shape in consequence 

 of a suggestion made by Mr. John Ware Stephenson, ... of 

 London, who independently discovered the principle of homogeneous 

 immersion.' 2 



This method consists of the replacement of water between the 

 covering glass of the mounted object and the front surface of the 

 object-glass by a liquid having the same refractive and dispersive 

 power as crown glass. With such a fluid taking the place of air, it 



1 On ' Stephenson's System of Homogenous Immersion for Microscopic Objec- 

 tives ' (Abbe), Jo-urn. R.M.S. vol. ii. 1879, p. 257. 



2 Ibid. 



