1889.] 



MICROSCOPICAL JOURNAL. 



51 



the objective is perfect. The mounter has on his table, for example, the 

 lenses of an apochromatic. The lenses of several apochromatics may be 

 indiscriminately mixed before him, but as they are all ground exactly to 

 measure, it is of no consequence which ones he puts together. He 

 picks up the separate lenses, which have never before been brought 

 together, puts on a drop of balsam, examines it with a hand-lens to see 

 that there is no dust, then presses on the other lens, and the work is 

 sure to be right. The lenses are then accurately set in their brass 

 mounting. The distances between their surfaces are measured with 



Fig. 2. — Zeiss Heating Oven. 



great care and made to conform exactly to the calculations. It is in- 

 teresting to know that when the first apochromatic, a 3 mm., was put 

 together it was perfect, proving that the calculations were right. The 

 second was not perfect, but it was found that the workman had made 

 a slight error in putting in the lenses. I believe he put one lens in 

 wrong side up. When this error was corrected the objective was right. 

 The instrument for measuring is graduated to 0.01 mm., but half of that 

 can be estimated by the eye. 



