ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 323 



suggested the advisability of making the standards of rock-crystal, instead 

 of glass, for testing Microscope lenses, on the ground of its much greater 

 durability where required to be in such constant use. Each workman was 

 also provided with a contact-measurer, by which he was able to determine 

 the thickness of the lenses, and thus approximate to the required thickness 

 within a small fraction of error. An experienced foreman superintended 

 this department, and was responsible for the accuracy of all gauges, &c. 

 Mr. Mayall said he had been much interested to see these methods of pre- 

 cision in regular daily use in Messrs. Zeiss's workshops, the more so from 

 the fact that for much of the optical work lads were employed, who thus 

 obtained admirable training for the more difficult branches on which they 

 entered later on. He had also witnessed the processes of centering the 

 separate lenses, and reducing them to the required diameters ; then the 

 cementing into combinations and the mounting in metal cells, with its 

 attendant further process of centering. He had also watched the whole 

 process of manufacturing a front lens for an apochromatic 1/8 homo- 

 geneous-immersion, from the grinding to the complete mounting in its 

 cell, centering, &c., the lens being somewhat greater than a hemisphere, 

 and the figure being tested in the standard concave of rock-crystal as he 

 had previously described. The rapidity and dexterity shown throughout 

 the execution of this delicate work had most favourably impressed him as 

 to the high character of the training in Messrs. Zeiss's workshop, for it 

 should bo noted that the production of such work was not confined to one 

 pair of hands, as generally obtained in England, but was being executed 

 by several — workmen of special aptitude, doubtless, but still such as 

 the system of training there adopted brought to the fore in sufficient 

 number to meet the demand, even in so large an establishment. He 

 had also observed with special attention the methods employed for testing 

 the finished objectives ; but there, of course, so much depended on the 

 education of the eye and judgment, that he could not venture to criticize, 

 not having himself practised with Prof. Abbe's silvered plate method. He 

 understood, however, from Prof. Abbe that the method enabled the director 

 of that department to give precise instructions as to alterations needed to 

 reach a certain standard of excellence. 



He must not omit to refer to the photomicrographic department, to 

 which Dr. Roderick Zeiss had given special attention. A separate building 

 bad been erected for this purpose, and massive concrete blocks supported 

 the installation of the electric light, projection apparatus, &c., as free as 

 possible from vibration. Here he had seen a number of images of test 

 objects, &c., projected on a screen by means of an arc lamp of 1200 c.p., 

 using various objectives, from 1 in. to 1/20 in. focus. In some instances the 

 higher degree of achromatism attained in the new apochromatic objectives 

 was unquestionably shown, and he had no difficulty in admitting that on 

 the whole the projection images were the best he had ever seen by artificial 

 light. In view, however, of the extreme difficulty — impossibility he might 

 say — of controlling the arc lamp, of maintaining a steady and equal light 

 even for a space of one or two minutes, he thought for purposes of photo- 

 micrography it could not be commended, especially not for producing large 

 negatives by direct projection. He had long held the opinion that the best 

 photomicrographs were obtained by making small negatives by direct pro- 

 jection, negatives just large enough to exhibit the points sought to be 

 demonstrated ; if, then, it were desirable to produce a further enlargement, 

 the small negative could be magnified by an ordinary photographic process. 

 In this way the best photomicrographs by Dr. Van Heurck, of Antwerp, 

 were produced, and the most difficult results, such as photographing the 



