ON THE MORPHOLOGICAL VIEW OF XATURE. 229 



cession of iiuproveiuents by whicli enois due to colour 

 and indistinctness — the chromatic and spherical aberra- 

 tions — were removed. In the middle of the century 

 the influence of some eminent Ijotanists, notably of 

 Hugo von Mohl and Nageli, in perfecting micrometric 

 processes was considerable ; whilst the last twenty 

 years have witnessed quite a new departure in the 

 theory of optical images, in that of microscopic vision, 

 in the improvement of optical glass, and in the in- 

 vestigation of the possible limit of the magnifying 

 powers. The most eminent physical authorities — such 

 as Stokes and Lord Rayleigh in England, Helmholtz 

 in Germany — have taken up one or more of these 

 points ; but the whole subject is associated with 

 the name of Prof. Ernst Abbe ^ of Jena, who, 

 through his connection with the well-known firm of 

 Carl Zeiss, has been able to put into actual practice 

 many of the suggestions which resulted from his the- 

 oretical investigations. As the historians of zoology 



^ The labours of Abbe go back to 

 the year 1873. Simultaneously and 

 independently, Helmholtz attacked 

 the theory of microscopical vision 

 and the question of "resolution " — 

 i.e., of the possible limit to the re- 

 solving power of any optical arrange- 

 ment. Airy had attacked the same 

 subject on purely dioptrical lines. 

 Helmholtz and Abbe went a step 

 farther, taking into account the 

 physical nature of light as a wave- 

 motion, subject to interference 

 phenomena, notably those caused 

 by inflection, where objects with 

 very fine markings are concerned. 

 Ablje's methods were for a long 

 time only imperfectly known. The 

 publication, however, of his theories 



by Czapski ('Theorie der optischen 

 Instrumente nach Abbe,' Breslau, 

 1893) made the whole subject better 

 known, and has been followed by 

 two masterly papers by Lord Ray- 

 leigh and Prof. Johnstone Stoney in 

 the 42nd vol. of the ' Philos. Mag.' 

 (1896). The latter paper especially 

 gives several interesting examples 

 of the use of recent microscopic 

 appliances and the means of avoid- 

 ing errors in handling very delicate 

 and minute objects. It seems that 

 the instrument caimot any longer 

 be used without a theoretical know- 

 ledge of its optical construction, 

 which enables the observer not only 

 to see, but also to criticise and to 

 interpret. 



