580 PROCEEDINGS OF THE SOCIETY. 



Mr. J. Mayall, jun., paid lie wished to correct and supplement 

 certain points in his de6cri])tion of Nobcrt's ruling machine as re- 

 IX)rtcd in tlio Proceedings of the April meeting. (1) The divisions 

 to 5 minutes in arc on the bands of silver imbedded near the circum- 

 ference of the division-plate were in all probability made by means 

 of a large circle-divider, and from them the twenty-one (not twenty) 

 rows of " dots " would have been made with great accuracy. The 

 divisions, therefore, should not have been referred to as supple- 

 menting the "dots," but as the prime factor of accuracy of the 

 dividing engine. (2) The stud and dots miglit have been used for 

 cutting toothed wheels, but not for ruling diffraction gratings or 

 interference plates. These latter would require the greatest possible 

 accuracy, and therefore llerr Nobert would assuredly adopt the most 

 certain method, which was to work with the division-plate and the 

 micrometer-Microscopes. The tangent-screw caused the division- 

 plate to rotate with great accuracy, so that Herr Nobert may not 

 have found it necessary to inspect the divisions by the micrometer- 

 Microscopes for each line ; he may have tabulated the error of the 

 tangent-screw, and made periodic allowance for it. (3) In order to 

 reduce the motion in arc when ruling test-plates, where the movement 

 of the test-plate represented the sine of the arc motion of the division- 

 plate, Herr Nobert would probably use the arc motion only for the 

 production of the bands of lines, and in moving the test-plate over 

 the blank spaces between the bands would utilize the fine micrometer- 

 screw attached to the agate plate fitted within the hollow steel 

 cylinder carrying the test-plate, by which the latter could be easily 

 moved 1/2000 in. (the average width of the blank spaces between the 

 bands), or even less. In this way he would reduce the total arc 

 motion to about one-half. To minimize this differential motion 

 Herr Nobert doubtless used the greatest possible excentricity of the 

 short arm of the lever, so as to utilize the smallest motion in arc 

 required for his subdivisions. 



The following Instruments, Objects, &c., were exhibited; — 



Mr. Beck : — " Star " Microscopes. 



Dr. J . D. Cox : — Photographs of broken diatom valves. 



Mr. Crisp: — (1) Bausch and Lomb Optical Company's mirror, 

 with diaphragms ; (2) Simple Microscope, with " multiplying glass " 

 as the illuminator. 



Mr. Groves : — Cambridge Eocking Microtome. 



Mr. Guimaraens : — Six slides of Pathogenic Bacteria. 



Dr. Maddox: — Slides illnstrating his paper. 



Mr. J. Mayall, jun. : — Model of Gray's Water Microscope. 



Mr. Wethered : — Slides illustrating his paper. 



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