242 



Rutherfurd Photographic Measures. 



ii. 



DIVISION ERRORS. 



The glass scale of the micrometric measuring machine is about 

 seven inches long, and has 335 spaces. All the measures of dis- 

 tance were made with this scale. It was read by means of a read- 

 ing microscope of the customary form, so arranged that ten revo- 

 lutions of the screw corresponded to one space on the scale. Thus 

 it was possible to read directly to .001 spaces, and by estimation to 

 .0001. All readings were made in this way to .0001 spaces. Before 

 the reduction of the observations was begun the scale was sent to 

 Professor W. A. Rogers, of Colby University, Waterville, Me. 

 The latter has investigated the division errors. The method 

 employed was not such as would free the results from cumulative 

 error ; but it is highly probable that a sufficient degree of accuracy 

 for the present purpose has been attained. Professor Rogers has 

 measured the 50-spaces, the 25-spaces, the 5-spaces, and finally the 

 single spaces, with his micrometric apparatus ; and the results are 

 given in terms of 1 division of his micrometer microscope. This 

 equals 0.0000002 meters. The measures of the 50-spaces have not 

 been used, as they were only made as a check upon the measure- 

 ment of the 25-spaces. The following are the results: — 



