PROCEEDINGS OF THE SOCIETY. 797 



the necessity of counting all the wave-lengths, and when the standard 

 yard had been calibrated it would be interesting to compare the results 

 obtained with those now accepted as correct. When the determination 

 of the standard yard was completed he would suggest that the Royal 

 Microscopical Society should submit a micrometer to the Board of Trade 

 to be calibrated, and provided that the temperature could be maintained 

 at the same degree, it would remove all sources of error, and would 

 provide them with a reliable standard of measurement. If the state- 

 ments in the paper before them could be trusted, the value of their 

 present rulings was far from being satisfactory, but if they had a stand- 

 ard to refer to, all uncertainty would be removed. Mr. Conrady's 

 suggestion that the Society should procure such a standard was an 

 excellent one, but he would suggest that no action be taken until they 

 were in a position to have one calibrated in terms of a fixed unit, such 

 as a wave-length of light. Two other questions had been raised in the 

 course of this discussion — one as to the deterioration of the glass by 

 keeping, and this was a point of importance because almost all glass used 

 for fine ruling was polished, by which the harder outer surface was 

 removed, and he should think that many of the micrometers made 

 a long time ago would be found to have been affected by lapse of time. 

 The other point was as to the shrinkage of glass by age, he thought there 

 was very little doubt that such did take place, and as micrometers made 

 thirty years ago were not made of specially aged glass, he thought it pro- 

 bable that some of them might have suffered from this cause. Mr. 

 Grayson's rulings were mounted in realgar, which would certainly pro- 

 tect the surface from deterioration so long as the realgar did not 

 crystallise. 



Mr. J. W. Gordon did not think he was in a position to give any 

 general answer to the questions raised, and he should shrink from doing 

 so in the name of Dr. Ewell, from whom he held no authority to speak on 

 his behalf. Referring to the question asked by Mr. Hopkinson whether 

 the error was all in oue direction or not, as in the case of thermometer 

 tubes, he observed that this was a point not covered by Dr. Ewell's 

 paper. The writer had only in a few cases gone into the question of 

 total error, his examination being for the most part limited to the uni- 

 formity of the divisions, whether right or wrong, so that total error did 

 enter into his purview. He (Mr. Gordon) had been greatly interested 

 in the paper and the questions it had raised, and especially so by the 

 suggestion of Mr. Conrady, as it seemed a very desirable thing that the 

 Society should possess a standard micrometer for reference and com- 

 parison. He thought anything in the way of appreciation of the paper 

 would perhaps be out of place as coining from him as the reader of 

 the paper on behalf of Dr. Ewell ; he would therefore content himself by 

 making those few observations. 



The Secretary read a requisition, signed by eleven Fellows of the 

 Society for a Special Meeting of the Society to be called for the purpose 

 of altering the By-laws in such a way as to remove the present prohibi- 

 tion of the attendance of AYomen at the Meetings of the Society. He 

 gave notice that part of the next Ordinary Meeting would be made special 

 for the consideration of this proposal. 



