PROCEEDINGS OF THE SOCIETY. 789 



Mr. W. Wesche's paper on " The Mouth-parts of the Nernocera, and 

 their Relation to the other Families in Diptera — with Corrections and 

 Additions to the paper published in 1904," was read to the Meeting by 

 Dr. Hebb, who explained that the portion now submitted was a resume 

 of the entire communication, and that he had been asked to communi- 

 cate it to the Meeting, as Mr. Wesche's health did not permit him to 

 read it himself. 



The Chairman said this was a very important paper, which would be 

 printed in extenso in the Journal, but it was, of course, difficult to judge 

 of its full value from the abstract which had been read that evening. 

 It would, no doubt, be read with considerable interest when they had it 

 before them. 



The thanks of the Society were unanimously voted to Mr. Wesche. 



Mr. "Wesche said that most of the points mentioned in the paper 

 were illustrated by specimens exhibited under Microscopes on the table. 

 The subject was one which recpiired a considerable knowledge of the 

 mouth-parts of Diptera to be able to appreciate thoroughly, although he 

 thought that anyone who had a Topping slide of the proboscis of the 

 blow-fly would be able at all events to understand one aspect of it ; but 

 instead of looking for minute hairs as test objects, workers with the 

 Microscope would find it a more profitable and delightful study if they 

 would give their attention to a comparison of the remarkable differences 

 in the mouth-parts of this very large order of Insects. 



Mr. E. M. Nelson's paper " On the Resolution of Periodic Struc- 

 tures " was read by Dr. Hebb. 



A further paper by the same author on " An Auxiliary Illuminating 

 Lens " was also read. 



Mr. J. W. Gordon said that the first paper struck him as being a 

 very suggestive one, and he thought the observations which Mr. Nelson 

 had carried out were of very particular interest, because they reduced to 

 a specific form what had hitherto been propounded only in a conjectural 

 way. The verification of these conjectures was therefore a matter of 

 very peculiar interest. One thing, however, he should like to suggest 

 in this connection for further consideration, because Mr. Nelson's 

 observations appeared to refer only to the half of the diffraction fringe 

 which overlaid the dark field. There is a complementary dark half 

 which overlies the edge of the bright field, and has much to do with the 

 obscuration of line structures. This was never investigated, so far as he 

 was aware, by any writer of authority, until it was taken up by Lord 

 Rayleigh, and dealt with in his supplementary paper published in the 

 Journal of this Society in 1903. When the bright field was narrowed 

 sufficiently, this dark fringe extended right across it and prevented the 

 illumination of this field from attaining to full intensity. This point, 

 although it had no direct bearing on the paper, was so closely connected 

 with the subject under discussion, and of so much importance that he 

 hoped it would not be regarded as an impertinence on his part to 

 mention it in this connection. 



Mr. Curties said that he was showing in the room a lamp fitted with 

 a lens as described by Mr. Nelson. 



