344 PROCEEDINGS OF THE 



revealed one or two difficulties in connection with the reading of 

 the instrument that is, if any accuracy in the second decimal 

 place is required. The first difficulty is due to the fact that in 

 Mr. Cheshire's instrument we have to interpolate or estimate 

 between two divisions on a scale, one of which is not visible, being 

 outside (apparently) the margin of the back lens of the objective. 

 This renders the estimation of the second place of decimals in the 

 N.A. uncertain,' and although Mr. Nelson's modification of the 

 original instrument is somewhat better in this respect, yet the 

 very means adopted to improve the reading namely, the intro- 

 duction of a large number of additional circles is likely to con- 

 fuse the diagram and bewilder the observer. In either the old 

 form or the new of Cheshire's instrument, a count has to be made 

 of concentric circles a thing which, simple as it may seem, is 

 peculiarly liable to confuse the eye, so that it is only after count- 

 ing several times that one feels certain that the number is, say, 

 eight, and not seven. 



Mr. Ainslie exhibited and described a new method of reading 

 the N.A. of an objective, 



The President said the Club was much indebted to Mr. Ainslie 

 for his communication, and also to Mr. Cheshire and Mr. Grundy, 

 to whom the thanks of the meeting were unanimously voted. 



At the 493rd ordinary meeting of the Club, held on Novem- 

 ber 25th, 1913, the President, Prof. A. Dendy, D.Sc., F.R.S., 

 in the chair, the minutes of the meeting held on October 28th, 

 1913, were read and confirmed. 



Messrs. W. M. Bale, B. Shepherd, H. Dobell, E. W. Ramsay, 

 M. R. Licldon, A. Panichelli, Robert Young, W. G. Tilling, and 

 E. J. E. Creese were balloted for and duly elected members of 

 the Club. 



The President read a letter from the nephew of the late Sir 

 Ford North, which was in reply to the vote of sympathy passed 

 at the last meeting. 



Mr. C. E. Heath, F.R.M.S., brought before the notice of the 

 meeting a device for preventing damage to objective or slide, 

 especially when the higher powers are used, in cases where the 

 microscope is liable to unskilful usage, as, e.g., at soirees. A 

 small piece of thin metal steel was suggested is taken, having 



