Obituary. 157 



Abbe had arrived at his results by studying afresh, from first 

 principles, the whole problem of microscopic images, from the 

 standpoints of both geometrical and physical optics. He came 

 to the task with a fresh mind, for till that time he had not 

 specialised in optics, nor had he worked much with optical instru- 

 ments. This, in the case of an intellect so keen and vigorous, and 

 fortified by the necessary training in physics and mathematics, was 

 a positive advantage, for no preconceived ideas hampered him in 

 drawing his conclusions from the extensive course of experimental 

 work which accompanied his theoretical investigations. It was in 

 the course of these that, following up the work of Fraunhofer on 

 the telescope, he evolved his famous Theory of Microscopic 

 Vision, based on his discovery of the modifications produced in 

 the image by the diffractive action of the object itself on the light 

 by which it is illuminated. This theory has been known chiefly 

 under the name of the Diffraction Theory — presumably because in 

 Abbe's original papers, to use his own words, " Different structures 

 always yield the same microscopic images as soon as the difference 

 of diffraction effect connected with them is artificially removed from 

 the action of the Microscope ; and that similar structures as con- 

 stantly yield different images when the diffractive effect taking 

 place in the Microscope is artificially rendered dissimilar." 



Abbe showed that the optical system of the Microscope — 

 usually considered as consisting of an objective to produce a 

 magnified, inverted image, and an ocular acting as a simple magni- 

 fying glass to enlarge this image — might be analysed in a different 

 way. He demonstrated that the Microscope system was equiva- 

 lent to a telescope with a loup, or simple magnifying glass — to 

 parallelise the rays from the object — placed in front of it. Thus, 

 the Microscope objective can be looked upon as consisting of two 

 lens components, one acting as a simple magnifying lens, behind 

 which the other, acting as a telescope objective, occurs ; the object 

 and its image being hi the principal focal planes respectively of 

 these two components. In conformity with this characteristic 

 analysis of the Microscope, Abbe treated the problems of micro- 

 scopic image formation in two steps : firstly, investigating the 

 nature of the light distribution in the back focal plane of the ob- 

 jective (which takes into account the diffractive action of the object 

 on the light source, according to the mode of illumination) ; 

 and, secondly, deducing the image in the image-plane from this. 

 This method clearly brings out the difference between the imaging 

 of self-luminous and non-self-luminous objects — a fact which has 

 led to Abbe's theory being also known, more especially on the 

 Continent, as the " Theory of Secondary Imaging." 



The theory was first given to the world in 1873, in a paper 

 entitled "A Contribution to the Theory of the Microscope and 

 the Nature of Microscopic Vision," in M. Schultze's " Archiv fur 



