50 THE AMERICAN MONTHLY [March, 



impression that the brass was blackened by chemical methods. When 

 the parts are put together they require to be adjusted by the most skil- 

 ful workmen, and no stand is passed until it is as perfect as skill can 

 make it. The machinery is run by two engines, of respectively 16 and 

 8-horse power. 



By far the most interesting part of the work is the glass grinding. 

 The glass is cut by means of a disk of metal revolving like a circular 

 saw, the edge charged with diamond dust, and turning in a well of pe- 

 troleum. It is astonishing to see how qinckly a thick block of glass 

 can be cut in this way. Even rock crystal does not seem to be very 

 hard when put against the wheel. The glass being cut into slices of 

 suitable thickness, the larger lenses are roughly rounded on a grind- 

 stone, and the curvature of the surfaces made to correspond with metal 

 patterns or matrixes. Then follows more careful grinding by boys. 

 Each boy sits at a table and grinds with a lathe and a brass matrix 

 with emery, testing the curves as he goes on. Five grades of emery are 

 used in this work. The final polish is given with rouge, in a matrix of 

 pitch and shellac mixed together.* Every lens is inspected by an ex- 

 perienced foreman before it leaves the room. 



Such, in brief, is the method of grinding all lenses, but for achro- 

 matics and for microscope objectives, more practiced workmen are era- 

 ployed and more rigid tests applied. Frauenhofer, many years ago, 

 proposed the use of quartz patterns to test the curvature of telescope 

 lenses, but this method was independently invented by a workman in 

 the employ of Mr. Zeiss about the year i860, and by him first applied 

 in the grinding of microscope lenses. It is now used throughout the 

 establishment for all achromatic lenses. Each workman has his quartz 

 pattern, which he keeps under a glass shade on his table. As the pol- 

 ishing goes on he cleans off the rouge from time to time, and tries the 

 lens in the joattern, being extremely careful lest the smallest particle of 

 dust or grit should get between them. The lens must fit the pattern 

 perfectly. If it does not, the workman can detect the error at once by 

 the feeling, or by the appearance of the colored rings, known as New- 

 ton's rings, where the contact is not perfect. These quartz test-patterns 

 have enabled the workmen to do the most perfect work — far more per- 

 fect than is possible by the old method of grinding — and it will be seen 

 as we go on, that some such accurate method for testing is required in 

 order to work strictly to the formulae calculated bv Dr. Abbe for each 

 objective. 



There is another feature of the grinding that is also of great impor- 

 tance, for, when the cui'ves are right, it is also necessary that the lens 

 should be of the exact thickness required. The workman has an in- 

 strument for accurately measuring the thickness of the lenses as he 

 grinds them. This is quickly done, for the device is very simple. 

 Each lens is finally measured and examined by the foreman of the 

 grinding-room, and when the lenses leave his hands they are ready to 

 be cemented together and put into the brass mountings. 



In these operations we see the results of the perfect methods of grind- 

 ing, for, although each single lens is ground by itself, when the different 

 lenses of an objective are brought together, and once set in their mounting, 



* There is a special polishing composition used for the finest glasses, which gives a more perfect 

 polish than rouge. 



