18 



If tlie exact value of Mr. Peirce's grating space was kuown, that of the grating 

 used in these measnreincnts could be at once ascertained with a degree of accuracy 

 equal to that of the angular measurement of this line and whenever the final com- 

 parisons are completed it will he easy to apply the necessary corrections. 



MICROMETRIC MEASUREMENTS. 



Micronietric measures were made in a few cases in which the lines were 

 so near tt) the reference line that the effect of a possible error in the value of the 

 micrometer screw or in the adjustment of the grating would be small. In order 

 to determine the value of the micrometer screw twenty measurements were made 

 of the distance between E and b„ this distance having been carefully ascertained 

 by means of the circle. The space between these two Hues was divided into 

 two nearly eipial parts which were separately measured, in order to bring the 

 measurements into the centre of the field as nearly as possible. The resulting 

 value of one division of the head of the micrometer screw was applied to the 

 measurement of h.^, b^ and 6„ referred to 6; and also to D., referred to D,. The 

 e.xtreme distance measured in this way was less than 11' and it is evident from 

 the investigation already given that the results will be free from error due to 

 grating adjustment and also that they would be but little influenced by tempera- 

 ture although the latter correction has been applied. 



RESULTS. 



The following are the results of the angular measurements made upon 

 what was presumed to be van der Willigen's. No. Iß, for the purpose of ascertain- 

 ing the relation of the grating to that of JMr. I'eirce alreadv referred to. 



As the result of this series may possibly be of great value in the final 

 adjustment of the absolute wave-lengths, it is thought proper to give all of the 

 original measures, together with the temperature at which each was made, and 

 the value of each when reduceil to the standard temperature. Th<' numbers 

 will also serve to give an idea of the general accurac)* of the angular measure- 

 ments as well as of the sufficiency of the temperature corrections. It ought to 

 be said, perhaps, that lor the most accurate angular work this line is not well 

 suited; a thinner lino would midoubtedly give a more uniform series. But the 

 fact tliat the line is easily seen, even under inferior optical power, is decidedly 

 in its favor, as far as its general utility as a "reference line" goes. The measures 

 were made in the spectra of the second order. 



