484 PROCEEDINGS OF THE SOCIETY. 



not yet done this, what else could one do than pick it up from wherever 

 one might he able to find it ? and so he was obliged to go to Naegeli and 

 Schwendener's book for it. Concerning the proposed use of a concave 

 lens, Prof. Thompson had said that he did not see that any good 

 would come of it. The speaker could not himself say whether or not 

 this was a practicable way of making an objective ; but at least an 

 opportunity had been given of ventilating the subject. He regretted 

 that Prof. Thompson had not been present to hear the beginning of the 

 paper, and hoped he might have the pleasure of showing him the ex- 

 periments which were arranged upon the table, by which he thought his 

 points had been demonstrated. 



As to Mr. Eheinberg's part in the discussion, he could only say 

 that the paper which he had launched upon them dealt with so many 

 matters that it would involve too long a time to go over them even if it 

 were possible to pick them up. There was, however, one point funda- 

 mental to Mr. Rheinberg's remarks — he said the error had been made 

 of ignoring the fact that with a plane wave-front they must always have 

 diffraction. In the first place he, the speaker, had not ignored it ; on 

 the contrary, he had pointed out that this was so, but he had also shown 

 that the crowding together of tbe diffraction spectra of which he had 

 spoken was due to the fact that curved wave-fronts were substituted for 

 plane wave-fronts in the plane of the grating, and Mr. Rheinberg had 

 only added the perfectly correct explanation that this was due to the 

 different phases of the light in different parts of the grating when the 

 wave-fronts passing it were curved. The same explanation afforded 

 an answer to Prof. Thompson's criticism of the experiment with the 

 aerial image of a ruled screen which in the paper was described as a 

 self-luminous object. It was self-luminous in the sense that the wave- 

 front in that object had a radius of curvature practically = 0. 



The President said he was sure they would give a very cordial vote 

 of thanks to Mr. Gordon for his very able and instructive paper. It 

 was no doubt somewhat difficult for them to follow throughout a critical 

 and expository paper full of details, and therefore difficult to intelligently 

 discuss it, but they would all have the opportunity of a more leisurely 

 study of it when they could read it in the Journal. 



A hearty vote of thanks to Mr. Gordon was then put from the Chair, 

 and carried by acclamation. 



The President announced that the meetings of the Society would now 

 be suspended for the summer vacation — the date of the next meeting 

 being October 16. He further intimated that the rooms of the Society 

 would be closed from August 16 to September 16. 



New Fellow.— Mr. E. G. Wheler was balloted for and duly elected 

 a Fellow of the Society. 



The following Object was exhibited : — 



Mr. Thos. H. Powell: — Coscinodiscus aster omplmlus under J^ in. 

 Apochromatic Oil-immersion Objective. 



