HERMANN VON HELMHOLTZ 



spherical surfaces, the properties of cardinal points, 

 and the theorems of Gauss relative to the refractive 

 powers of centred systems of spherical surfaces, all 

 matters relating to what may well be called physical 

 optics, are discussed, so that, in forming an estimate 

 of all that Helmholtz has accomplished in this depart- 

 ment of science, the treatment of these subjects in a 

 work mainly physiological must not be forgotten. 

 Nine papers, however, are enumerated in his collected 

 works as specially belonging to physical optics. Five 

 of these relate to colour, and might well have been 

 included in the list of fifteen works on physiological 

 optics, so that only four remain. One of these, 

 published in 1867, is a mathematical essay dealing 

 mainly with the action of lenses; two, issued in 1873 

 and 1874, deal with the higher optical principles of 

 the compound microscope and the limits of magnifica- 

 tion ; and the last, which was also issued in 1874, 

 relates to the theory of anomalous dispersion. Finally, 

 in 1892, there appeared a paper, important from a 

 theoretical point of view, in which he applied Clerk 

 Maxwell's electro-magnetic theory of light to explain 

 the dispersion of colour. 1 



In this connection, and as it is a matter of practical 

 interest, it may be pointed out that Helmholtz was 

 often in the habit of magnifying small movements 

 by reflecting a beam of light from a small mirror 



1 All the papers on light appear in Bd. ii. Wisienschaftl. Abhandlungen ; 

 that on colour dispersion in Bd. iii., s. 505. 

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