248 Mr. Louis Bell on the Absolute 



method appears to be of somewhat questionable utility. It 

 avoids, to be sure, the necessity of placing the grating normal 

 to the axis of either telescope ; but as there is very little 

 trouble in making this adjustment with a high degree of 

 accuracy, and keeping it through a series of measurements, 

 the gain is by no means considerable. Aside from this ques- 

 tion, Mascart's spectrometer read only to five seconds ; and 

 while his results with different gratings agree very well indi- 

 vidually, they are certainly collectively in error by quite a 

 large amount, very possibly owing to bad standards of length. 



It is a fact to be noted in discussing all these earlier wave- 

 length determinations, that sufficient attention was not paid to 

 the measurement and study of the gratings — by all odds the 

 most difficult part of the problem. The angular measurements 

 of any one of the above investigators were good enough to have 

 given very exact results had they been combined with proper 

 investigations of the grating-spaces. As most of Noberfs 

 gratings were small and by no means accurately ruled, there 

 was peculiar need of care in measuring them ; and when one 

 considers that the defining-lines on most standards of length 

 are far from being good, it is clear that the chances of error 

 were numerous. In Angstrom's first [taper he even relied on 

 the grating-space assigned by the maker. Ditscheiner em- 

 ployed a grating which had belonged to Fraunhofer himself; 

 but the number of spaces was uncertain, and this led to a large 

 error, which he corrected, in part, in a supplementary paper 

 some years later. Ditscheiner's principal paper was published 

 in 1866; and was followed in 1868 by an elaborate discussion 

 of the whole problem by Van der Willigen, whose paper is 

 valuable mainly for a particularly elaborate review of sources 

 of error. Like his predecessors, he used Nobert's gratings; 

 but as the construction of his spectrometer confined his an- 

 gular measurements to the deviation on one side of the normal, 

 their accuracy may be somewhat open to question ; while his 

 standard of length was anything but reliable, as it was a glass 

 scale only three centimetres long, and the only assurance of 

 its accuracy was the certificate of the maker that it was " tres 

 exacte" at 50° Centigrade. For one or both of the above 

 reasons, Van der Willigen's results were larger than any which 

 have been obtained, before or since his time. 



In the same year appeared Angstrom's great research which 

 has so long served as the standard in all questions of wave- 

 length. It is hard to say too much of the conscientious and 

 painstaking experiments on which his results were based ; and 

 any want of accuracy in the final result was due to no lack of 

 skill or care on his part, but rather to the imperfect instru- 

 ments with which he was obliged to work. Like every one 



