177 



plied which would not dissolve, the ring would entirely obviate the danger of 

 decomposition from contact with the air. 



Mr, William Webb read a paper upon Nobert's Tests, illustrating the subject 

 by drawings upon the black board. 



The President said he was quHe sure that all the members of the Club would 

 feel very much obliged to Mr. Webb for his paper ; it opened up a great many 

 thoughts upon the subject, and the explanation given of the cause of the 

 shadows seen in the closer bands seemed so simple that it must carry conviction 

 with it. 



Mr. Webb regretted that great pressure of engagements had prevented him 

 from going near Lis workshop for the last fortnight, so that he was unable to 

 bring with him any specimens in illustration of his remarks. He hoped, how- 

 ever, to be able to do this at their next gossip night, and would then endeavour 

 more fally to explain his meaning, 



Mr. Ingpon thought that many of Mr, Webb's observations would be endorsed 

 by those persons who had used Nobert's lines as a test for the defining power of 

 their objectives. He had himself tried them for that purpose, and had given 

 them up, and his conclusions coincided very nearly with those of Mr. H. J. 

 Slack as given by that gentleman in a paper on *' Optical appearances of cut 

 lines in glass," which was read before the Royal Microscopical Society, and 

 printed in the " Monthly Microscopical Journal" for May, 18/1. Of course in 

 looking at the coarser bands of Nobert the furrow of each line, the angle, and 

 the two edges were perfectly plain, as was also any little dirt which might have 

 been upon the edges, but this clearness soon vanished when they came to examine 

 the higher bands, and he thought that Mr. Webb's suggestion as to the reason 

 for this was very likely to be near the truth. Even when the higher bands had 

 been resolved it was not in the same satisfactory manner as in the case of the 

 lower ones. With whatever clearness they might be shown, there was always a 

 row of lines on each side blacker and clearer than any of the others — an 

 appearance which he thought was seen in some of Dr. Woodward's photographs, 

 and which possibly were defraction lines, and which the last edge shown on Mr. 

 Webb's diagram was quite capable of producing. As to the spectral lines, he 

 fancied that Mr. Webb was slightly in error in referring them to polarization, 

 because there was, he thought, nothing either in the substance or the angle to 

 polarize the light. The effect was, however, very possibly due to diffraction ; it 

 was, in fact, similar to that produced by the ruled gratings such as were ex- 

 hibited at the soiree. The light was, he thought, decomposed and separated 

 into its component colors by interference caused by diffraction. The spectral 

 lines seemed due to the formation of an image either above or below the focus, 

 but they did not appear to be in any degree the result of polarization. Of course 

 the question as to tests, and as to what was the best, and whether these would 

 be so when blackened in, was one still open for settlement ; if the whole surface 

 of the glass (supposing it to be cut away as shown in Mr. Webb's diagram), were 

 to be blackened in, that at least would hardly constitute a test. For his own 

 part he could not imagine that a test could be obtained either for chromatic 

 or spherical aberration either by straight lines ruled on glass or by irregular lines, 

 such as letters, and he must say that his strong impression was that none of the 

 tests at present used, whether the globules of mercury, or scales, or even 

 the vexed Podura, were entirely satisfactory. He thought that they were all 

 very much indebted to Mr. Webb for his paper, and for the many valuable sug- 

 gestions contained in it. 



