565 



PROCEEDINGS OF THE SOCIETY. 



— r<l^\9i 



MEETING 



Held on the 28th June, 1911, at 20 Hanover Square, W., 

 H. G. Plimmer, Esq., F.R.S., etc., President, in the Chair. 



The Minutes of the Meeting of May 17th were read and con- 

 firmed, and were signed bv the President. 



The following Donations received since the last Meeting were an- 

 nounced, and the thanks of the Society voted to the donors : — 



From 

 Edward A Birge and Chancey Juday, The Inland Lakesj 



of Wisconsin. The Dissolved Gases of the Water. \ The Authors. 



(8vo, Madison, Wisconsin, 1911) ) 



The Secretary of 



the Committee 



of the Projiosed Optical 



Convention of 1912. 



Diatomaceous material for distribution Mr. C. F. S. Bilbrough 



The Proceedings of the Optical Convention of 1905 

 (8vo, London, 1905) 



Mr. Conrad Beck gave the following demonstration of the method 

 of determining in wave-lengths the measurement of a stage-micrometer. 



Of late years several papers have been read before the Society setting 

 forth the results of a series of extremely careful and laborious measure- 

 ments of different specimens of stage-micrometers. Such measurements 

 have all been comparative, showing the differences that exist in various 

 ruled micrometers, but have in no case been measurements of the 

 absolute value of the rulings. Where ruled micrometers are found 

 which are uniform, and where a number of micrometers made by 

 entirely different methods agree, there is a probability that such micro- 

 meters may be approximately accurate, but it does not follow that such 

 is the case. 



Since Professor Micbaelson by means of an interferometer measured 

 in wave-lengths the French metre, a great deal of attention had been 

 paid to this method of determining and recording measurements of 

 length, and Major MacMahon and Dr. Tatton are applying a similar 

 method to the measurement of the British standard yard. 



A wave-length of light being a very small quantity of about 

 3Fouo i n -j a number in the neighbourhood of two millions of these small 

 units are contained in the metre, and the determination of the exact 

 number requires very elaborate apparatus and an indirect method of 

 procedure. 



A stage-micrometer, however, ruled to ^^ in. or T ^o mm. contains 

 a comparatively small number of these units, and being ruled on a 

 transparent glass plate, is peculiarly well adapted for being measured in 

 this way. 



By means of an interferometer which my firm has made to the 



