322 SUMMARY OF CURRENT RESEARCHES RELATING TO 



RoYSTON-PiooTT, G. W. — Microscopical Advanccs. XVI. 



[Ancient and modern diffraction lines.] £ngt. Mech., XLV. (1887) p. 1. 



S., T. F.— See Nelson, E. M. 

 Taylor, J. T. —Photographic Lenses. 



[Contains remarks ou the new glass by the author, J. Mayall, jun., and others.] 



Journ. Soc. of Arts, XXXV. (1887) pp. 192-201, 268-9, 



(7) Miscellaneous. 



A Visit to Jena. — At the January meeting of the Society, Mr. J. 

 Mayall, jun., gave an account of his recent visit to Jena, where during about 

 a fortnight he had been the guest of Prof. Abbe. Every facility had been 

 given him for following the technical processes employed in the manufacture 

 of Microscopes in Messrs. Zeiss's optical and mechanical workshops, and 

 in the production of optical glass in the Jena Optical Glass AVorks, and his 

 impression was that it would be hardly possible to overrate the skill in 

 organization there displayed for the purposes in view. Messrs. Zeiss 

 employed upwards of three hundred assistants in a series of workshops so 

 arranged that those departments where delicate work was being produced 

 — where the vibration of steam machinery would be a serious drawback — 

 were quite separate from the departments where steam-power was employed. 



Messrs. Zeiss had found it advantageous to make their own brass castings, 

 and hence had established a foundry on their premises. He had seen the 

 various heavy kinds of lathe-work and fraising in full operation with steam- 

 power. The parts of the Microscope-stands where this and other mechanical 

 work was being executed were usually given out in sets of ten, and in general 

 the system of piecework was in vogue throughout the workshops. With 

 regard to the optical work, only a very small portion was produced by 

 the aid of steam-power ; for instance, the plane surfaces of eye-piece lenses, 

 which were worked together in large sets, and the glass-slitting by means 

 of rapidly-revolving iron discs charged on the edges with diamond frag- 

 ments. The glass-slitting machine was largely employed in the preparation 

 of prisms of the different samples of glass for the determination of the 

 refractive and dispersive indices. By means of the glass-slitter, the plates 

 of optical glass, as received from the glass works, were cut to the various 

 thicknesses required, and then, by means of ordinary American wheel- 

 cutters, the thin strips were cut into squares of the sizes required. The 

 squares were placed in suitable trays in the storeroom, whence they were 

 given out to the glass-grinders, together with the necessary tools and the 

 gauges belonging to them. The glass-grinders snipped the squares to 

 approximately the disc shape, and then cemented them each on a suitable 

 block, and ground and polished the surfaces, the metal tools being attached 

 to foot-lathes with vertical spindles passing through deep horizontal trays, 

 in which the refuse emery, &c., was caught, and the workmen were 

 generally seated. 



For testing the accuracy of the finished surfaces, Fraunhofer's method 

 was employed, which consisted in providing for each curvature required 

 a pair of highly-finished standard convex and concave surfaces worked 

 in rock-crystal, of which the radii had been accurately determined by 

 means of a spherometer of great precision, the perfection of the curva- 

 tures being shown by the symmetrical formation of Newton's rings when 

 the surfaces were pressed in contact. Each surface, as finished, was tested 

 by contact with the corresponding standard surface of rock-crystal, and the 

 polishing was continued until the required degree of accuracy was reached. 

 He was previously aware that Fraunhofer had employed this method of 

 testing the accuracy of spherical surfaces for telescopes, using standards 

 made of glass. Prof. Abbe informed him that Dr. Hugo Schroder had 



