PROCEEDINGS OF SOCIETIES. 179 



this position were the only ones from which an object-glass could be 

 corrected and constructed. As regarded the question of angular 

 aperture, he felt sure that there had been much mistake about it. No 

 doubt, as he was one of the parties in the dispute, it would not do for 

 him to pass judgment by measuring the glasses of different makers in 

 contradiction to their own lists, but it appeared to him that many 

 mistakes had been made because a great deal of false light entered at 

 extreme incidences. This could be easily shown, and he was sui-e that 

 oftentimes a mistaken increase of aperture was due to such false light 

 and not caused at all by image-forming rays. By placing a minute 

 stoj), with its front plane, exactly in the focal point of the object-glass, 

 this might be demonstrated, as such a stop would prevent the entrance 

 of false light beyond the range due to true aperture, and possibly 

 from this cause the same glasses bad sometimes been found to give 

 such widely different measurements in a short range. In one or two 

 instances he had already found that with this safeguard attached, the 

 entire range given by the maker, between " covered " and " uncovered," 

 did not show a difference in the angle of aj)erture, or but a very 

 slight one. By due investigation such a condition could probably be 

 accounted for. 



Mr. Slack supposed there was a real change, but that the false 

 light interfered with it. 



Mr, Wenham assented to this, and said that the question was of so 

 much importance that he thought of making it the subject of a com- 

 munication to the Society, and would then bring an instrument by 

 which it could readily be tested, and thus furnish a demonstration at 

 the next meeting. 



The Secretary thought that as Mr. Stephenson had worked with 

 Dr. Pigott's instrument, perhaps he might be able to give them his 

 opinion of it. 



Mr. Stephenson said he was afraid he was not in a position to 

 offer any observations. He had certainly used the searcher, and he 

 quite agreed with what Dr. Pigott said about its being very difficult to 

 use. If required merely to increase magnifying power, it would be 

 found much more useful than any eye-piecing. 



Mr. Slack would only say that he hoped some Fellows of the 

 Society would pay attention to Dr. Pigott's method of testing 

 objectives ; he did not presume to give an absolute opinion upon it, 

 but having seen it in operation for many hours on various glasses, he 

 believed it would enable them to make a reliable quantitative com- 

 parison. It was, he thought, a matter of much importance, especially 

 to those who made good objectives. 



Dr. Pigott said he had come up from Reading to attend that 

 meeting, in order to thank the President for the handsome manner in 

 which he had referred to him in his Annual Address, but he had seen 

 with great surprise a letter in the last number of the Journal, in 

 which it was stated that the President, in his Addi'ess, said the 

 searcher was a wholly fictitious thing upon its merits. 



The President, interposing, said that his remarks on that occasion 



