PEOCEEDINGS OF THE SOCIETY. 569 



escaped detection it would be of the greatest benefit to the race as well 

 as of its present high scientific interest. 



Mr. Beilby thought a slight correction was necessary, as it appeared 

 to be thought by some speakers that this was an absolutely new method 

 of observing the particles in ruby glass and in ruby-gold solutions. 

 Faraday satisfied himself forty years ago that the colour was due to the 

 presence of minute separate particles, and one of his methods of verifying 

 this was by concentrating a beam of sunlight through the ruby material, 

 and in this way it was made evident that there were minute particles of 

 gold distributed throughout the glass or solution. 



Mr. Beck said it was so seldom that they were favoured with the 

 presence of Dr. Czapski that he thought it would be greatly appreciated 

 if he would inform the Meeting what was, in his and his colleagues' 

 opinion, the effect of Mr. Gordon's work upon the Abbe theory of 

 microscopic vision. In his opinion it was impossible to reconcile the 

 two theories, and Mr. Gordon's work was a direct attack upon the 

 correctness of some of Prof. Abbe's theory. 



Dr. Czapski said he was not at all prepared to go fully into the sub- 

 ject of Prof. Abbe's theory, which would take a great deal of time, but 

 Mr. Beck was quite right in saying that Mr. Gordon's first paper had 

 created the impression upon readers not fully informed that Prof. Abbe's 

 theory was wrong, and so it would be useful to go once into the details 

 of that paper. At Jena they had received copies of the paper in June 

 1901 and had occupied themselves in examining it, but they found 

 nothing in this very able paper which icas in contradiction to the Able 

 theory. The unfortunate thing was that Abbe had never published 

 the full development of his theory, nor had others done so. Prof. 

 Zimmermann's book, alluded to by Mr. Rheinberg, was only a very 

 rough popular sketch of the real theory. This, as he had just said, was 

 given more completely in Prof. Abbe's lectures since winter, 1887-8. 

 If one desires to give an approximate idea of his meanings in- 

 telligible even to the non-skilled mathematician or to examine a 

 general theory by experiments, he must use only the most simple 

 conditions, as in this case take for the objects, lines or points (single 

 or double), edges of screens, or periodical structures, for instance 

 gratings. But it would be a thorough misunderstanding of Prof. 

 Abbe's work to think that he has given only a " resolution-theory " or 

 a "grating-theory" of the Microscope. He has treated the problem 

 in its most general form : given certain luminous points arranged on a 

 surface, given another surface (a layer of infinitely small thickness) in 

 which absorption and refractive index vary after any law ichatever — 

 how can the movement of the ether behind that surface be determined 

 as to phase and amplitude, more especially, how can it be reduced to 

 some most general laws ? Further on — and this leads to the special case 

 of microscopical vision : if behind the before-named infinite thin layer 

 (the object) a system of lenses be arranged, of which this layer forms 

 one of its aplanatic foci — what general indications can be given for 

 the distribution of the light in the other aplanatic plane (the image 

 plane) ? These investigations, as repeatedly said, were most general 

 with the only restriction that the object is of a (comparatively) very 



