10 



of the Royal Microscopical Society, of his intended visit to Jena, he (Dr. 

 Dallinger) at once expressed his earnest desire that every objective in his 

 collection that might enable Dr. Abbe to correctly estimate the quality 

 of English optical workmanship should be placed at his disposal. Mr. 

 Frank Crisp, Secretary of the Eoyal Microscopical Society, also desired that 

 his collection of objectives should be examined by Dr. Abbe, and similarly 

 with Mr. E. M. Nelson, who had spared no efforts to collect examples of 

 the best known objectives. Keferring to his visit to Messrs. Zeiss's Optical 

 Works at Jena, Mr. Mayall said upwards of 300 persons were there em- 

 ployed, and the organisation appeared to him excellent throughout. He 

 had witnessed the various technical processes of manufacturing microscopes. 

 In the mechanical departments he had examined everything from the brass- 

 casting in the foundry to the final adjustment through which every micro- 

 scope stand went before being sent away from the workshops. In the 

 optical departments he had watched the process of glass-slitting, cutting 

 into squares, &c., grinding, polishing, gauging of thicknesses, testing the 

 accuracy of the sphericity of the surfaces, centering, cementing into com- 

 binations, mounting in cells — in fact, the building up of objectives on a 

 very large scale, where the system of manufacture passed them successively 

 through the hands of more and more highly-trained workmen until they 

 reached the final stage of testing to standard before being regarded as 

 finished. For testing the sphericity of the surface, Fraunhofer's contact 

 method was employed, which consisted in utilising for each curvature a 

 corresponding gauge or test surface, finely and accurately worked in 

 rock-crystal, the contact of the two surfaces in various positions displaying 

 Newton's rings more or less symmetrical!}', according to the accuracy of 

 the surface under trial. The accuracy of the rock-crystal gauge itself was 

 in eveiy case assured by its own corresponding gauge, also of rock-crystal, 

 the contact of which showed perfect symmetry in the formation of Newton's 

 rings. In practice the surfaces of the lenses were polished and tested with 

 the rock-crystal gauges until the requisite degree of accuracy was attained. 

 He had observed with particular attention the manufacture and mounting 

 of the front lens of an apochromatic one-eighth homogeneous immersion 

 where the spherical curve was somewhat greater than a hemisjihere. The 

 skill with which that delicate piece of work had been executed had given 

 him a high opinion of the system of training in vogue in Messrs. Zeiss's 

 establishment, for it should be noted that even such high-class work was 

 not limited to one pair of hands, several experts being wholly engaged upon 

 similar work. With regard to the Jena Optical Glass Works, Mr. Mayall 

 said they were under the direction of Dr. Schott, and appeared to him 

 planned with admirable practical aims. He understood the installation was 



