23 



E 



This is a double, one line helw^ due to ir.ni and tlie otlier to calcium and 

 iron. The measurements were made upon the latter, which is that of the great- 

 est wave-length. It was upon the E line that Ang.«troin made his most 

 careful measurements aud he inesents in full, in his memoir, the series of 3.5 

 oliservations. But these appear to have been made upon the mean of the two 

 lines, at least the result is exactly the mean of the wavc-lensrths of the two lines 

 given in his tahle and in liis series of micrometric mcasiirei? values difTering 

 slightly from these are a.ssumed. It is a little difficult to undci-stand how 

 accurate angtdar measures could he m.ade upon the mean of two lines, or, at 

 le.-ist, wliy one of tliem shoidd not have been selected, provided there was suffi- 

 cient dispersive power to separate them, which must certaiidy have been the 

 ciu^e. It is wortli noticing that the wave-length obtained by Angstiöm for the 

 mean of these lines agrees precisely witli that of the line of greatest wave-length 

 as determined bv the followiuo; measures. 



Date 



No. 



of Obs. 



Value of i. 



/ 



Nov. 26 





3 



45° 



51' 9".l 



52G!t.l3 



J» i> 





3 



J) 



„ 8'.1 



„ .10 



Nov. 27 





4 



)j 



„ 9".8 



„ .14 



7J >> 





3 



)) 



„ 9".8 



„ .14 



Tt '» 





3 



»» 



„ 9".5 



*„ .14 



?I )> 





3 



?j 



„ 9".4 



„ .13 



" . " 





3 



n 



„ 9".l 



„ .13 







Mean of 22 



observ 



ation. — 









/ =: 



ViO'.l.i: 



5 





/<! /<_, /i; and /', 



.•\s regards these lines the nonienelature of Angstrom's table ami ina)) has 

 lieen followed. According to Angstrom ainl Tlialcn, A, and /«. an- to Ik- attributed 

 to niau'ncsimn, l>^ tn nii-kcl and iron and />, to uiai^nesinni ami imn. 



Tlie lirst. i,, is ii broad line or liaiid but near tin- ed;x'' towards the blue is 

 a narrow, well dclined line upon whicli the setting wiw made. The distance from 

 h, to K K'ing less than 1 ■* the |>ositioii of the line wiw determined in reference to 

 K the measures Iw-ing made, however, with the eireli- and not with tlu^ micro- 

 meter. It is U'lieved that no error existed in thi' iuljustment of the grating of 

 siifiicient magnitude to influence tlie n-snlt«, es|HM-ially as the grating was read- 

 justed Bcvcral times during the measurement.'«. 



