QUEKETT MICROSCOPICAL CLUB. 97 



variation in pollen grains, and recommended members to study 

 them. Dr. Hartridge was then called upon to read his paper on 

 " Microscopical Illumination." Dr. Hartridge said that the ex- 

 periments described were 8 or 9 years old, and that his con- 

 clusions to some extent had been anticipated by Mr. Ainslie, 

 with whom he was in close agreement, in a paper read before 

 the Photomicrographic Society. The theory of microscopic 

 vision held at first by the majority of microscopists was 

 Fraunhofer's, further elaborated by Airy and Helmholtz. 

 This theory stated that the image consisted of antipoints 

 surrounded by systems of rings, and the object was regarded 

 as behaving as if self-luminous. Critical illumination origin- 

 ated with the desire to use the substage condenser under 

 the best possible conditions, the luminosity being greatest 

 when the light was focused on the object. There were others 

 who used oblique light and hollow cones which appeared 

 to give better images of diatom structure with the objectives 

 then available, especially for photography. This latter 

 school received Abbe's theory with acclamation as the last 

 word against their rivals who believed in the Fraunhofer 

 theory, and so a violent controversy began. A few months later 

 Dr. Spitta published his book not only supporting whole-heartedly 

 the principle of critical illumination, but strongly inclining to the 

 theory of Abbe. Dr. Spitta reconciled the two views by pointing 

 out that Nelson's solid cone was nothing more than a number of 

 narrow Abbe beams placed side by side, and that the diffracted 

 beams could be shown to accompany the direct beam through 

 the upper lenses of the objective. Dr. Spitta's views were 

 not coincident with those of his contemporaries, and different 

 interpretations were possible of Abbe's scrappily published 

 theories. Dr. Hartridge criticised the statement that the chief 

 difference between the rival theories of microscopic vision is 

 that Fraunhofer starts from the object (regarding it as self- 

 luminous), while Abbe starts from the source of light. He 

 advanced evidence to support the view that the " source " of 

 Abbe's theory is usually a virtual source placed at infinity of 

 such dimensions that the train of plane waves illuminating the 

 object appears to be proceeding from it. After considering 

 microscopical illumination from the point of view of the Abbe 

 theory, Dr. Hartridge went on to examine the position of critical 



