538 



formula which they had heard quoted for the resolving limit 

 applied to a grating which is absolutely uniform in ruling, and, 

 .secondly, to a grating also which has an infinite number of lines 

 that is, practically, if 60,000 to the inch, there should be many 

 inches so ruled. When a grating has a limited number of lines, 

 it becomes easier to resolve. Grayson's 50 lines at 60,000 is in 

 this way subject to a 3 per cent, discount. No grating is really 

 uniform, and the " ghost " spectra ha.ve also been called into use 

 to help resolution. Referring again to the Grayson rulings, Mr. 

 Conrady said that we have only Mr. Grayson's word as to the 

 number of lines ruled. Some time ago Mr. Merlin published in 

 the Journal R.M.S. an account of a series of measurements he had 

 made of these fi.ne rulings. He found that the different spaces in 

 one band varied as much as 10 per cent. But Mr. Merlin did 

 not attempt to measure whether the 60,000 band was really 

 60,000. He only compared it with the 10,000 band, and we have 

 no knowledge as to the accuracy of the 10,000 band. There is no 

 standard of reference for such things. Di*. Spitta had recently 

 measured some coarse rulings over one inch by Mr. Grayson. 

 These showed a periodic error in the dividing, and, further, the 

 inch divisions did not agree in value with a similar space divided 

 into metric values. He believed, however, that Mr. Grayson did 

 not claim that these rulings were absolutely correct. The coarsest 

 line in a group will influence the resolution of that group. 



Mr. Stokes said that he was quite satisfied with the undulatory 

 theory, and also that it would be quite equal to all demands made 

 upon it by microscopists. 



After the meeting Mr. Stokes exhibited the 60,000 band of a 

 Grayson ruling resolved with an objective of N.A. 0*835, and a 

 dark-ground illuminator of N.A. 1*34. 



Mr. R. W. H. Row, B.Sc, who was introduced by the President, 

 said that some ten days ago some curious objects were found on a 

 platform at Maiden railway station in Surrey, and were brought 

 to King's College. The appearance at Maiden was as if the whole 

 of the gravel of the platform was jerking about in a series of 

 spasmodic movements. The sycamore trees overhanging the 

 platform showed in many cases brown patches on their leaves. 

 On being held up to the light, a little black spot was seen in the 

 brown patch. This proved to be a larva, between the upper and 

 under sides of the leaf, feeding on the soft mesophyll. It probably 



