PROCEEDINGS OF THE SOCIE'lY. 507 



as the sodium light is not a purely monochromatic light, but consists of 

 two lines which are not in themselves quite monochromatic, sodium does 

 not form a very good source for the illumination. Either cadmium, 

 mercury or hydrogen, used in connexion with a prism which shall direct 

 one only of the chief lines into the instrument, is what is best for the 

 purpose. 



The micrometer is held on a bracket which projects out from the 

 carriage, on which the moving mirror is fixed in such a manner that, 

 although it is viewed by the Microscope, it is not in contact with it, and 

 no friction is caused which could interfere with its motion. 



The President, in inviting comments, informed the Meeting that on 

 the following day, at the Eoyal Society, a paper would be given on the 

 use of silica as a standard for measurement. That fact would show that 

 the standard of measurement was not yet settled. 



Mr. Conrady said he thought the chief interest in comparing 

 standards of length with the wave-length of light lay in comparing 

 standards themselves, the mixed standards, because when it came to small 

 things like stage-micrometers, it was of no use to those who wanted to 

 measure ten-millionths, and he doubted whether any method could de- 

 termine them. What was wanted was constancy of materials for the 

 standard length, for one part in a million and less. Xo one would want, 

 for practical purposes, to know nearer than one part in a million ; one 

 part in ten thousand was what most people would be interested in. He 

 thought that if, at the present day, one were to send a stage-micrometer to 

 the National Physical Laboratory at Teddington, they would, in a very 

 short time and for a very few shillings, state what was the value of the 

 millimetre, or whatever measurement it might be on that micrometer, to 

 a far greater degree of accuracy than one was likely to want for measur- 

 ing bacteria or infusoria, because all these forms of life were grow- 

 ing things, and so there was no constancy about them. It was a 

 different matter with standards of length, because very few materials 

 were even fairly constant now, and probably the platino-iridium which 

 was used in the international metres was not quite reliable. He reminded 

 the Meeting that there was no such thing as the French metre now, 

 there was only the international metre, because the Institute at Paris 

 was now the International Bureau of Weights and Measures, and the 

 metre which they issued was internationally accepted by Great Britain, 

 as one of the signatory nations ; so that England was bound to the 

 Paris metre, and anything which might be done or said at this Society 

 could not alter the international standard of weights and measures. 

 There was doubt about that constancy, and after very lengthy experi- 

 ments, the International Commission adopted a 10 p.c. platino-iridium 

 as the most unchangeable material which they could find, and in that 

 material the international metres were made. In the choice of material, 

 quartz was considered, but being a crystalline substance there was some 

 doubt about it. It was important to remember that Michaelson and his 

 successors had adopted another standard, which was fairly above sus- 

 picion as regards constancy. It had been pertinently pointed out that 

 the whole solar system was travelling through space at the rate of, he 

 believed, something like 20 miles a second, and was therefore encounter- 

 ing a different supply or area of ether every second, and very sensibly 

 so. Therefore in the course of a few days there must be possibility of 



