30U 



Mr. Ingpen thought the members would be very glad to have the oppor- 

 tunity of seeing and comparing these various plans, for it was particularly 

 the province of societies like theirs to deal with things of this kind. No 

 doubt the American plan was very pleasant to use, but he thought Mr. 

 Nelson's new device was a great improvement upon his former plan, and at 

 the same time had the very particular advantage that it need not cost very 

 much. 



The thanks of the meeting were unanimously voted to Mr. Nelson and 

 Mr. Curties for their communications. 



Mr. Nelson read a paper " On the Cause of some Errors in the Interpre- 

 tation of Small Microscopical Objects."* 



Mr. Ingpen said that the question of entoptic shadows was one of much 

 interest and importance. The lines seen round an object as described 

 by Mr. Nelson were analogous to the diffraction lines seen round the 

 images of stars in a telescope. 



Mr. Michael thought he had understood Mr. Nelson to say that the cone 

 of rays arising from a condenser should be suited to the angle of the 

 objective, and also to say that the No. 6 hole was suited to most objectives, 

 these two statements seeming to be at variance. 



Mr. Nelson said this raised a most interesting question. He had no doubt 

 that if they could have an absolutely perfect objective they would get the 

 best performance from rays which came at the same angle as that of its 

 aperture, but in practice this could not be realized, because the marginal 

 zones were not the same as the central. 



Mr. Ingpen thought that Dr. Fripp did not mean, so far as he knew, that 

 the whole aperture of the condenser should be used, but that it must have 

 an aperture equal to that of the objective, so that light of any required 

 obliquity might be obtained. No doubt Mr. Nelson's idea was a new and 

 important point of departure. 



Mr. Michael did not think that Dr. Fripp contended that the whole 

 aperture should be used, but rather that the aperture of the condenser 

 should vary according to that of the objective ; that was that they should 

 be suited to one another. 



The thanks of the meeting were voted to Mr. Nelson for his commu- 

 nication. 



The President announced that the meeting was favoured that evening 

 by the presence of Dr. Josiah Curtis, of Washington, to whom, in the name 

 of the members, he gave a hearty welcome. 



Dr. Curtis said it afforded him the greatest pleasure to meet them on 

 that occasion. The Quekett Club was so well known that he was most 

 happy to bear his testimony to its reputation in America for the life, energy, 

 and activity with which it pursued its subjects of investigation. 



The President announced the meetings and excursions for the ensuing 

 month, and called special attention to the whole day excursion to Whit- 

 stable on June 2nd and to the excursionists' annual dinner on June 30th. 

 Members were also reminded that at the next ordinary meeting, nominations 



* This paper has been unavoidably delayed, but will appear in our next issue.— Ed. 



