HERMANN VON HELMHOLTZ 



conception of pairs of oppositely charged particles 

 (ions) of inert matter fixed in the ether. This paper 

 probably contributed more to supporting the electro- 

 magnetic theory of light than to an adequate ex- 

 planation of anomalous dispersion, which is an easily 

 demonstrable fact still incapable of explanation. 



It was in 1873 and 1874 that Helmholtz wrote two 

 important papers on the Optical Principles of the 

 Compound Microscope, and determined the limits of 

 amplification. This was accomplished independently 

 of similar and even more elaborate work, both mathe- 

 matical and experimental carried out by the greatest 

 living authority on all such questions, Professor Abbe of 

 Jena, to whose researches, especially in connection with 

 the construction on correct principles of apochromatic 

 lenses, science owes so much. Helmholtz, in the first 

 place, established a formula by which we can express 

 the ratio of the linear magnitudes of the object and its 

 image, in terms of the divergence of the rays before 

 and after refraction, which is independent of the 

 distance of the focal lengths of the refracting surface. 1 

 He showed also how to measure and define the 

 angle of aperture, and finally proved that in conse- 

 quence of the dispersion of light at the edges of 

 minute bodies, no objects can be seen that are smaller 

 than the -g^V^h of a millimetre, that is, the y^s^oo^ 

 of an inch. So far the microscope can go and no farther. 



1 Heath's Geometrical Optics, p. 56. 



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