22 



3Iean of 40 observations— 

 / = 5894.780 



Forty measurements were ma'le with the micrometer for the distance of Do 

 from Dj. The lines are too wide for very exact measurements but the distance 

 between their centres thus obtained is doubtless sufficiently exact. 



It is 



o = 4' 46".8 ± 0".2 



from which we have 



h = 53° 18' 50". 3 



and / = 5888.68 



From these results it appears that 



D — D, = 6.10 



According to some previous measurements already referred to this quantity 

 is as follows: 



Mascart 



6.3 



Ditscheiner 



6.4 



van der Willigen 



•6.14 



Angstwim furnishes two results diflfering slightly from each other. From 

 his tables of wave-lengths it appears that the distance between these lines is 

 6.01, but on another page of his memoir he determines the wave-lengths 

 of the two lines according to what he calls the "method of coincidences ' and 

 from those values we have, 



D.— n, = 6.08 



These last nuinbei-s he evidently considers the most accurate for he adopts 

 their mean for his wave-length of D in his final table of "definitive values". 

 The result given above differs very little, therefore, from what may be accepted 

 as Angstrom's best measurements. It is impossible, of course, that any error 

 will be found to exist in the grating space which will affect this result in the 

 second decimal place: but a small error in that figure may exist consequent 

 upon errore in the niicrometric measures. 



