PROCEEDINGS OF THE SOCIETY. 251 



obligation to Mr. Beck for the loan of apparatus for the purpose of the 

 demonstration, and to Mr. E. Russell Clarke — a stranger to the Society — 

 who had kindly lent him the Microscope which they saw upon the table, 

 fitted with an electrically driven screen. Mr. Clarke unfortunately was 

 not able to attend that evening, and he (Mr. Gordon) had not succeeded 

 in getting the Microscope to work properly. He hoped, however, on 

 some future occasion Mr. Clarke would be able to attend to give the 

 Fellows a demonstration of the working of his very beautiful instrument. 

 With reference to Mr. Gifford's inquiry as to the displacement of the 

 screens, in the case of Mr. Russell Clarke's arrangement, the oscillation 

 was perfectly regular, describing a long ellipse, the magnitude of the 

 longer axis being about three times that of the shorter, the long axis 

 measuring about r ^o- in. In the case of the other screens exhibited on 

 the table, Fellows would be able to see the motion for themselves as one 

 of them was working opened up. Care had been taken to provide against 

 any chance of a repeated pattern. The screen was moved in a circle 

 by a ring embracing it, but was so much smaller than the ring, that it 

 rolled within the ring, the result being a sort of epicyclic motion. He 

 thought Mr. Rheinberg would find there was no difficulty in getting rid 

 of all trace of the screen, in photography ; and in answer to Mr. Beck, he 

 said that he had fully committed himself to the exposition of the sine-law 

 — if that were in any respect erroneous it could be criticised. He should 

 be very sorry to do any injustice to Prof. Abbe, but had not attempted 

 to trace the original statement of the sine-law. The only thing with 

 which he was concerned in that paper was the proof of the law, and the 

 proof of it was admittedly due to Helmholtz, except for Hockin's defec- 

 tive proof which was discussed in the appendix to the paper. Mr. 

 Conrady had taken exception to the term "antipoint." That was a 

 matter which he would not discuss as Mr. Conrady was clearly entitled 

 to his opinion on such a point. With reference to the opinion which 

 Mr. Conrady had expressed that the sine-law only applied to a small 

 element of surface lying on the optical axis, he suggested that if that 

 were his view Mr. Conrady would do well to read. Helmholtz' paper. It 

 was no doubt possible to have an optical system in which the tangent 

 law took the place of the sine-law. That was, however, all dealt with 

 in the appendix to the paper, and as it was written out there it would 

 not be necessary to refer to it further. Mr. Rheinberg complained that 

 nothing was said about the optical properties of the object : that was 

 because Helmholtz said nothing about them. Much might no doubt 

 be said about the object, but Helmholtz in his paper treated not of the 

 object but of the instrument interposed between the object and the eye. 

 It was not to be supposed that an oscillating screen could increase the 

 resolving power of an objective. What he did was to preserve the 

 integrity of the image formed by the objective when greatly super- 

 amplified by eye-piece magnification. So with regard to the use of an 

 oscillating screen in photomicrography. The finely resolved image on 

 a large scale could be obtained by means of a projection ocular and an 

 optical bench. The object of using the screen was to get rid of these 

 cumbersome accessories, and obtain the high magnification at a short 

 distance from the stage by means of a compounding draw-tube. 



