476 PEOCEEDINGS OF THE SOCIETY. 



crossing one another at right angles, thus forming inclosed interspaces 

 for showing the production of intercostal points, as well as of colour 

 fringes, by the overlapping of the spectra. 



Experiment No. 11. — A spot of light was viewed through various 

 apertures, circle, square, long oblong, to show the form of the anti- 

 point images produced by apertures of those different shapes. 



The President said the Fellows of the Society were to be greatly con- 

 gratulated upon having had the opportunity of hearing the extremely 

 interesting explanation of this theory which Mr. Gordon had given 

 them. It was not only of much theoretical interest, but, if it were firmly 

 established, it would lead to a very material modification in the manu- 

 facture of objectives. 



Prof. Silvanus Thompson, F.R.S., being called upon by the President, 

 said he had not come there with the slightest intention of saying any- 

 thing upon the subject of the paper; for although he had been favoured 

 with an advance copy, he regretted that he had not had time to read it, 

 and he also further regretted that he had been unable to be present in 

 time to hear the reading of the first portion of it. But though not prepared 

 for this reason to discuss it, he might say that on one point at least he 

 entirely concurred with Mr. Gordon, and that was in rejecting the presenta- 

 tion of the Abbe theory as given by Naegeli and Schwendener ; but on 

 almost every other point he found himself at variance with Mr. Gordon, 

 and, so far as he could judge, the central facts of the theory remained abso- 

 lutely untouched by the arguments which had been put before them. He 

 did not quite see why it was necessary to draw a distinction between 

 a refraction and a diffraction image, because they could not have a 

 refraction image without the light, and they could not have either 

 without having interference. It seemed to him to be rather a question 

 of terms and what was understood by them. If by diffraction was 

 meant those effects on the image which took place where a wave-front 

 just grazed along the edge of an object, he could only say that he did 

 not regard this only as diffraction, because he considered that diffraction 

 might happen at any part of a wave-front, and was not confined to 

 anything which happened at the edge. On page 2 (of the printed slip) 

 (p. 355) he read, " It is a general rule that all the rays given out by a 

 luminous point which fall upon and pass through a properly corrected lens 

 are re-united in the focussed image of that point produced by the lens. 

 A ray of diffracted light differs for this purpose in no respect from any 

 other ray of light." He did not agree with this statement as a complete 

 description. He had in his possession a small diffraction object — one 

 of those curious sets which showed rings produced in celluloid films 

 made by Prof. Wood, of Wisconsin — this object merely acted by dif- 

 fraction, and it had no edges whatever. If this was put in front of a 

 point of light, it had the effect of retarding by certain definite amounts 

 the wave-lengths of the light in passing through, and it consequently 

 produced a real image of the luminous point, and in fact acted exactly 

 like a lens. It could not be said that the light here was refracted in 

 some way, because in one sense there was no refraction at all ; and the 

 fact that a diffraction image could be formed in that way by an object 

 of this kind did not surely prevent them from applying the same 

 principle to the Microscope. He did not think that the fact of the 

 beam being broken up by a grating prevented them from icgarding it 



