378 PKOCEEDINGS OF THE SOCIETT. 



plates barilly exceeded 1/50 in., this difference would bo inap- 

 preciable. 



The arrangement for carrying the diamond point was, be be- 

 lieved, wholly designed by Ilerr Nobcrt, and was a most ingenious 

 combination of mechanism, combining inventiveness and efficiency. 

 The questions to be solved were (1) To provide means to adjust 

 a diamond edge to any angle (within required limits) ; (2) to balance 

 it truly so that the weight-pressure for ruling might be perfectly 

 controlled ; (3) to raise and lower it strictly in one plane — that is to 

 say, mechanically free from lateral play, so that the consecutive 

 divisions of the ruling depended solely on the motion imparted to the 

 glass plate by the dividing engine ; (4) to cause the diamond to 

 oscillate freely in one plane ; (5 ) to control the length of the lines to 

 be ruled ; (6) to connect the whole with mechanism to ensure an even 

 rate of speed in the ruling movement of the diamond. These matters 

 had been workedoutby Herr Nobert with extraordinary perseverance, 

 as evidenced by the elaboration of the adjustments. From the point 

 of view of so accomplished a mechanician as Dr. Hugo Schroder — to 

 whom he (Mr. Mayall) must publicly cxj^ress his obligations for 

 exiilanations connected with the machine — the design of this portion 

 of the machine might be much simplified without loss of accuracy or 

 delicacy of action. In particular Dr. Schroder noted that the whole 

 of the movements might be made to act automatically, whereas Herr 

 Nobert went through the process of adjusting the dividing engine and 

 winding up the train of wheels for each line ruled on his test-jilates — a 

 labour demanding prodigious patience. It should be observed, how- 

 ever, that Herr Nobert had always to work with very limited means. 

 The success of his efforts must be estimated by those who were familiar 

 with his ruled jjlates, and who had compared them with others. 



For the production of difli'actiou gratings and ordinary micro- 

 meters, where the equidistance of the lines was an essential factor, 

 and where the breadth of space covered by the lines was so largo 

 that the lever motion in arc of the dividing engine would have 

 introduced errors in the evenness of the division, Herr Nobert 

 removed the bent arm from the centre of the division-j)late and 

 substituted a vertical cylinder, on which he coiled an extremely thin 

 flat steel spring having a hook at the free end ; this hook was attached 

 to a stud beneath a straight cylinder of polished steel, which carried 

 the glass plate to be ruled under the diamond, and which took the place 

 of the more delicate arrangement for the test-plates. The rotation 

 of the division-plate caused the vertical cylinder in the centre to 

 rotate, coiling the steel spring, and thus, after the manner of a wind- 

 lass, hauled along the diffraction-plate carrier at right angles to the 

 ruling motion of the diamond. Presumably Herr Nobert used the 

 stud and " dots " of the division-plate to divide the diffraction plates 

 and micrometers, for the verification of 12,000 consecutive divisions 

 (of which some of his diffraction gratings consisted) by means of the 

 Microscope, would have been too much for even his i^atience. 



Mr. Mayall then referred briefly to the preparation of the glass plates 

 for the rulings, which he said were of specially " mild " composition. 



