250 PROCEEDINGS OF THE SOCIETY. 



constant habit became automatic and involuntary in the same way as 

 Mr. Gordon had independently obtained it by mechanical means. 

 Nature has contrived this oscillation of the eye in order that a second 

 cone may receive the same image before the same impression has died 

 out from the first cone, and again a fresh impression on the first cone 

 before the impression produced by the second one had died away, so that 

 in this way the combined stimulus of several cones may suffice to convey 

 the impression to the brain which the feebler action upon one cone would 

 be insufficient to effect. 



Mr. Rheinberg said they had been hearing a great deal about the 

 formation of images so far as the optical system was concerned, but 

 nothing about the part played in this respect by the object itself, which 

 after all was important. Supposing they had isolated objects of no- 

 appreciable depth, such as bacteria, then there might under suitable 

 illumination be a considerable analogy between them and luminous 

 points or lines. If, however, they had objects having a certain depth 

 and consisting of separated elements, then they had entirely different 

 conditions whereby the image of certain of the elements might be modi- 

 fied by adjacent ones. 



Referring to the limit of separating power, was it not a difficult 

 matter to make out the case for anything less than half a wave-length ? 

 In speculations of this nature they required to take physiological con- 

 ditions into consideration. It was partly a question of the sensitiveness 

 of the eye to different degrees of luminosity, and this was also affected 

 by the size of the surfaces compared with the very small distance sepa- 

 rating them. If the surfaces were bright and relatively large, the eye 

 would i not distinguish the interval between them in the same way as if 

 isolated points were in question. He had noticed in the diagrammatic 

 illustrations of overlapping antipoints on the screen, that these had been 

 represented as of the same luminosity throughout, whereas of course in 

 reality they fell off very greatly in brightness towards their edges, which 

 would materially modify the illustration. 



As regarded moving screens in the focal plane of the eye-piece, he 

 inquired of Mr. Gordon, whether and what means were taken to ensure 

 the various parts of the screen moving at an equal rate, because if 

 revolving simply, the edge parts of the screen would be moving so much 

 faster than the central parts. Whilst he much admired the ingenuity 

 of the idea of the moving screen, he could not follow the utility of the 

 plan, because they could not thereby get any greater resolving power 

 than they had in the objective to start with. It appeared to him that 

 what was proposed, could be managed much more easily : for to obtain 

 the same sized image, and avoid any disadvantages connected with the 

 smallness of the Ramsden circle, they had only to use a lower eye-piece 

 and move the photographic screen further away. He should be very 

 pleased to hear Mr. Gordon's opinion on these matters. 



Mr. Gordon said he desired to thank the Fellows of the Society for 

 the kind reception they had given to his paper and to acknowledge the 

 indebtedness he felt to them for their patience in listening to his some- 

 what lengthy remarks. In particular he wished to acknowledge his great 



