PROCEEDINGS OF THE SOCIETY. 477 



as possessing the same properties as before, but it certainly would not 

 be refracted to the same focal positions as if there were no grating. 

 Then upon page 361 there was a passage concerning the images seen in 

 the Microscope of a Diffractions Platte — which concluded with the 

 words, " To tbese questions again there can be no answer except that 

 the Abbe theory was wholly at variance with the facts." He thought 

 on this point all depended upon what they meant by the Abbe theory, 

 and that if tbey took out some of the statements which had been made 

 with reference to it, there was not much in the effects themselves which 

 were at variance with it. Indeed, taking those beautiful experiments 

 which had been described, as to turning round the grating, they did not 

 seem to him to contradict tho theory, but were rather just what should 

 take place if it were correct. Then again ho did not agree with wbat 

 was said as to self-luminous objects, because he thought the term was 

 not correctly applied. For instance, if they had a number of fine 

 platinum wires stretched parallel, like the lines of a grating, and made 

 them white-hot by an electric current, they would properly be called self- 

 luminous, but they would give rise to no diffraction image, because the 

 light from them would not be all in the same phase ; but if they were 

 to be illuminated by a single light, it would be possible for them to 

 produce diffraction effects, because they would then be in phase with 

 one another. He also noticed that the effects described were referred to 

 a particular part of the wave-front, but be held that every point of a 

 given wave-front could give out the same kind of effects, because they 

 were in the limited sense of the term self-luminous, inasmuch as every 

 element of a wave-front was, in accordance with the well-known principle 

 of Huyghens, capable of acting as an independent source of light. He 

 did not think they would get much further by discussing the matter on 

 these lines. Taking the case suggested by the diagram last shown, as 

 to the construction of the front lens of an objective, he thought that if 

 there was really any advantage such as stated by making the front 

 slightly concave, it must surely have been tried by some one and most 

 probably rejected. 



Mr. Julius Eheinberg said the subject was one in which he had been 

 specially interested, and he should like to make a few remarks upon it. 

 As, however, he would be better able to express himself in writing, he 

 asked to be allowed to read his comments on Mr. Gordon's paper : — 



As one of those for whom the problems of microscopic vision have 

 a peculiar fascination — problems which I have been studying and ex- 

 perimenting upon a good deal myself during a number of years — I 

 should like to avail myself of the privilege of passing a few remarks 

 on the exceedingly interesting paper to which we have just been 

 listening. 



It is always refreshing to hear a subject treated in a new way, and 

 though, since the Abbe theory came out, there has been no lack of 

 valuable contributions to the subject of microscopic vision in this 

 country, as also in Germany and America, Mr. Gordon has, I think, 

 approached the subject from a new standpoint. 



The felicitous expression " antipoint," to denote the image of a 

 point, emphasises in itself the fact that the image of a point is not 

 another simple point, but a disc with rings, and that the image of a 

 line is not another simple line, but a line flanked with appendage lines ; 



