766 PEOCEEDINGS OF THE SOCIETY. 



contrast, and had nothing to do with resolving power. If they were to ■ 

 look up Lord Rajleigh's paper on the subject, which Mr. Gordon claimed 

 as his authority for his views, they would find that Lord Rayleigh 

 unreservedly adhered to the accepted limits of resolving power estab- 

 Ushed by Helmholtz and Abbe, and was therefore entirely at variance 

 with the interpretation put upon his paper by Mr. Gordon. 



Professor Porter said he had very little to say as to the paper, except 

 perhaps to emphasize Mr. Conrady's remarks, especially as to the 

 possibility of a single globule being utilisable as a test object. 



Mr. Beck said that he had been recently examining living bacilli with 

 high powers by means of a modification of the Siedentopf apparatus ; it 

 was extremelv interesting to notice the great difference there was in some 

 of the bacilli and minute organisms according to their reflecting cha- 

 racter, some of which were probably micrococci, showed as extremely 

 brilliant luminous spots surrounded by a brilliant series of diffraction 

 rings, which were so large in diameter and so numerous that they could 

 scarcely be caused by the diffraction of the Microscope object-glass, and 

 might probably be due to a reflection from a spherical surface in a similar 

 manner to those shown by Mr. Gordon as reflected from a mercury 

 globule. Other forms of bacteria showed no diffraction, and the reflected 

 light would appear to come from the marginal envelope of the bacillus. 



Mr. Gordon said the only point left for him to deal with seemed to 

 be as to whether a single object could be a test of resolving power. 

 What Mr. Conrady and Professor Porter had said on this point would be 

 perfectly true in the case of a single bright object, Init a single dark 

 object was for this purpose entirely unlike a bright object. A dark 

 object is itself invisible although it might in a sense be said to be seen 

 when in a bright field l)y contrast with the field in which it lay. Thus, 

 for example, a telegraph wire projected against a bright sky could not 

 itself be seen. An observer could not tell whether it was bright or rusty 

 or painted, but he might be able to see that the sky itself was divided 

 into two bright fields by a thin dark line. This is what people meant by 

 saying that they could see a telegraph wire in the sky, and thus the 

 visibility of the wire afforded evidence that the image of the sky was 

 divided up into two perfectly resolved parts. Thus, although a single 

 bright object affords no test of resolving power, a single dark object 

 seen by contrast with a bright field in which it lies is not only a possible 

 test, it is the only possible test and measure of the resolving power of 

 any optical instrument. 



Mr. E. M. Nelson's paper in reply to Professor Porter's and Mr. 

 Everitt's criticism of his paper " On the Limits of Resolving Power in the 

 Telescope and Microscope " was read by the Secretary. Professor Porter 

 said he had seen Mr. Nelson's paper and had written a reply, which he 

 proceeded to read, illustrating his remarks by diagrams drawn upon the 

 board, sliowing that the origin of the dispute appeared to be that Mr. 

 Nelson failed to realise that two objects might be so close together that 

 the central disks of their images considerably overlapped, yet there would 

 be so much less light in the middle that they would appear as two. The 

 usual conventional limit of resolving power corresponded with the case 

 where the first dark ring of one coincided with the central maximum of 



