PROCEEDINGS OF THE SOCIETY. 619 



Mr. J. Mayall, jun., described Dr. Schroder's new polarizing 

 apparatus. 



The President inquired if the apparatus had been worked out 

 theoretically only, or whether it had been attempted in practice ? 



Mr. Mayall said the analyser had really been cut and prepared 

 according to the formula described, although it had not yet been 

 applied to the Microscope. There could be no doubt as to its prac- 

 ticability, as the images were certainly most fully separated. 



Prof. W. A. Rogers' note to Mr. Crisp, accompanying one of 

 his standard micrometers, was read as follows : — 



" Will you please present to the Society on my behalf the 

 combined English and Metric Standard Micrometer which I have 

 forwarded to your address. It consists of 1001 lines in 1 centimetre, 

 1001 lines with the interlinear space l-2500th in., and of a repetition 

 of these bands with finer lines for high powers. 



The process of graduation was conducted in the following way : — 

 The error of the screw for one decimetre at 62° Fahr. was first deter- 

 miued by a comparison with the first decimetre of a standard metre 

 upon a bar of glass presented to me by Mr. Chaney, the Warden of 

 the Imperial Standards. 



The amount of this correction was then introduced into the 

 setting of the magnet-arm which governs the required part of a 

 revolution of the screw. If therefore no error was introduced through 

 the screw itself, the resulting centimetre should be really Standard at 

 62° Fahr. 



The band of lines 1001 to the centimetre was first ruled, the time 

 required being a little over an hour. The ruling carriage was then 

 set back to the starting-point and moved forward upon the ways such 

 a distance that the beginning of the first line of the next band should 

 fall upon a point near the end of the first line of the preceding series. 

 This operation was repeated with each band ruled. 



It will be seen that the first lines of the bands form one and the 

 same line, that is, this line is made up of four lines ruled indepen- 

 dently, and at difi'erent times. By measuring the distance between 

 the end lines of the first two adjacent bands it will be easy to deter- 

 mine the relation between the yard and the metre which was employed 

 in the graduation. 



I have recently subjected the centimetre of this plate to a test 

 which will add much to the value of the unit employed. The U.S. 

 Bureau of Weights and Measures has prepared a standard centimetre 

 upon a platinum-iridium surface. The lines upon this surface are of 

 the most beautiful character, and admit great precision in measure- 

 ment. This plate has recently come into my possession for examin- 

 ation, as a member of the committee for whom it was made. I have 

 compared it both with my bronze and glass standard centimetres. 

 The results are not yet fully reduced, but it is safe to say that 

 this centimetre is a very small fraction of a mikron too long at 

 62° Fahr. 



