616 SUMMARY OF CURRENT RESEARCHES RELATING TO 



to the right eye, according to the observer's custom of using either 

 eye habitually or both in succession. It is equally applicable to 

 stands whose construction does not admit of its being slipped over the 

 tube from the top ; the spring ring at the right of the figure being 

 in such cases made partly open so as to spring on from the side. 



Endomersion Objectives.* — Prof. K. W. Zenger claims to have 

 found that perfect achromatism of telescope and Microscope objectives 

 is possible by using a mixture of ethereal and fatty oils, the dispersive 

 power of which for the different rays of the spectrum increases regu- 

 larly. The disadvantages of the use of fluids are obviated by mixing 

 with suitable salts of the fatty acid series by which nearly hard or 

 gelatinous, vitreous, homogeneous, colourless, and transparent sub- 

 stances are obtained. 



The following are extracts from two papers published by the 

 author : — 



The construction of achromatic objectives for telescopes, M'oro- 

 scopes, and photography has, from the beginning, presented great diffi- 

 culties theoretically as well as practically. The dioptrical formulae 

 which give the equations for the achromatism and aplanatism of the 

 objectives are so complicated that, up to the time of Frauuhofer and 

 the younger Herschel, opticians were content with developing the 

 conditions of achromatism and aplanatism in the axis. In this way, 

 however, a perfect objective was theoretically not to be obtained, and 

 therefore the best makers of that time were obliged to confine them- 

 selves to experimental trials. 



Herschel and Fraunhofer first showed the way to a more accurate 

 determination of the direction of the rays, and the former has given 

 us a complete theory of telescope objectives, but for the much more 

 difficult computation of Microscope objectives almost nothing has 

 been done, and we to-day still look for a theory of these objectives. 



The principal practical difficulty for all kinds of objectives lies in 

 procuring suitable refracting media, because the flint and crown glass, 

 hitherto exclusively used, deviate greatly from the conditions of 

 perfect achromatism. Blair, at the end of the last century, showed the 

 possibility of getting rid of all colour by the use of at least three 

 refracting media, crown glass, oil of turpentine, and naphtha, which 

 give contrary secondary spectra, the dispersive power of one being 

 greater in the red, and of another in the violet part of the spectrum. 

 In this way he succeeded in making an absolutely achromatic 

 objective, the aperture of which was particularly large, namely, one- 

 third of the focal length. 



After Blair the matter was lost sight of until the second decade 

 of the present century, when Barlow made an objective of crown 

 glass and a biconcave lens filled with bisulphide of carbon on the 

 dialytic principle. The achromatism of this was not, however, 

 perfect, Blair's use of more than one fluid not having been attended 

 to, and the question again fell into oblivion. 



* SB. K. Bohm. Gesell. Wiss. Prag, 1881, pp. 479-92, 467-79 (reversed iu 

 order). 



