147 



The thanks of the Club were unanimously voted to the donors. 



The following gentlemen were ballotted for, and unanimously elected membera 

 of the Club :— Mr. Herbert Barnard, Mr. William K. Bridgeman, and Mr. 

 Charles E. White. 



Mr. Wm. Webb read a paper ** On the Best, the most Simple, and Unerring 

 Tests for Objectives." (The paper was published in the " Journal " of the 

 Q.M.C. for January, 18/3.) 



The President having expressed the pleasure with which he had listened to 

 the reading of the paper, and his sense of the indebtedness of the Club to Mr. 

 Webb for bringing the subject before them, invited discussion upon the points 

 which had been touched upon. 



Mr. Ingpen said he was sure that the members of the Club would welcome a 

 new test, though their criticism was at present disarmed, as they had not yet 

 had an opportunity of examining the objects which had been described, so as to 

 enable them to judge of their value as tests. Until they could do this, he 

 thought, they would have to fall back upon old tests, such as the Podura scale 

 and the P. angulatum ; for although we did not perhaps quite understand their 

 structure, we did know what errors they would test, and how a good objec- 

 tive should shew them. There was much difference of opinion as to the value 

 of Nobert's lines as tests, and he should be glad to have Mr. Webb's opinion, 

 and to learn his method of using them for that purpose. The value of the test 

 objects in common use consisted chiefly in the extreme regularity of their mark- 

 ings, in form and arrangement, and it seemed to him that such irregular objects 

 as letters or words, or isolated dots of carbon, however small they might be, 

 could not possess an equal value. He referred to Mr. Slack's experiments with 

 colloid silica,* as shewing the illusive appearances presented by irregular 

 fissures in a transparent substance. Mr. Webb had certainly introduced a new 

 unit of measurement— so many Bibles to the square inch — which might be 

 useful, though he (Mr. Ingpen) did not like the use of square measure. The 

 smallest spepimen was on the scale of 15 Bibles to the square inch, and he only 

 remembered this minuteness to have been surpassed in the case of the Lord's 

 Prayer written with the Peter's machine on the scale of 22 Bibles to the square 

 inch, as mentioned in the President's address to the Microscopical Society in 

 1862. Lines ruled on glass have for many years been used as a test for flatness 

 of field. We were apt to forget old methods of testing, and it was well that we 

 should be reminded of them. Part of the distortion referred to by Mr. Webb 

 was due to the construction of the Huyghenian eyepiece, and exists when the 

 spherical aberration of the object-glass has been well corrected and com- 

 pensated. If this were not done, the lines of a stage micrometer would not be 

 in focus at the edges of the field ; but when corrected, and the lines all fairly in 

 focus over the whole field, the line forming a diameter of the field would appear 

 straight, but the others, though really straight and parallel to it, would ai^pear 

 to curve outwards, the curve increasing as the lines approached the edges of 

 the field, the spaces between them also increasing in equal proportion. This was 

 the case with all Huyghenian eyepieces, however well constructed, and this was 

 the reason why they could not be used for astronomical measurements, as they 

 did not give equal areas throughout the field, and a Ramsden positive eyepiece 

 was used for that purpose, the objection to which was that it gave a highly 

 coloured image. Mr. Browning had, he believed, made a positive achromatic 

 eyepiece which met both difficulties. To test an objective for flatness of field a 

 * " MontMy Microscopical Journal," January 1, 1871, p. 14. 



