Lieut. J. Hersclicl on the Eclipse of August 1868. 839 



mula 86 , 2II + 1140, which refers the readings to another scale 

 and zero. 



In the following column are shown Mr. Pogson's readings of 

 the solar lines, of which those in italics are bright-line measures*. 

 The dark-line measures correspond with converted measures in 

 the previous column ; and the close agreement shows that the 

 empirical formula is correct, and that the dispersions of the two 

 instruments are commensurable throughout. Mr. Pogson's dark 

 solar lines are also unmistakably identifiable with 13, C, D, b, F, 

 and another, unnamed line, instead of those whose names he has 

 assigned. 



In the fifth column, those in the fourth are converted by the 

 empirical formula 4'40P — 6207, which refers the readings to 

 the scale and zero of that part of KirchhofPs map in which the 

 bright lines must be placed. 



The sixth column is derived from the second by the formula 

 4-40(86-211 + 1440) -6207, or 379-311 + 129, and is merely 

 a check on the identity of the lines supposed to have been mea- 

 sured. 



The seventh column shows KirchhofPs measures. Compared 

 with the two previous ones, it is evident that the empirical for- 

 mula? by which they are obtained are only applicable strictly to a 

 small portion of the spectrum — as was to be expected. 



The last columns show the positions on KirchhofPs scale of 

 all the bright solar lines of which measures are available, with 

 their (temporary) reference letters. 



Mr. Pogson's data would be more valuable had the dark solar 

 lines been measured immediately before and after the event ; but 

 he assures the writer that there was little, if any, change of zero 

 to be detected. He is also quite confident of the accuracy of the 

 bright-line measures. 



It is very remarkable that the red line H« was quite unseen. 

 Equally noteworthy is the evident preeminent brilliance of the 

 green lines (measured) which he describes as very bright, 

 although (owing perhaps to distressed eyesight) he was uncon- 

 scious at the time of their absolute colour. 



No green lines have yet been seen here with an uneclipsed 

 sun, although II 7 is frequently seen. Unless, therefore, greatly 

 increased dispersion can be brought to bear, future eclipses must 

 be depended on for the identification of these lines, whose exist- 

 ence has been vouched for by four observers of the late memorable 

 one. 



* [Sic in MS. The italics indicated are evidently the measures corre- 

 sponding to Pi and F 2 in column 9. — J. F. W. II.] 



