Satellites of the Mercury Lines. 445 



■effects so far as T can see. though the relative intensities of 

 some of the satellite lines appears to be a little different from 

 that which obtains when the lio-ht is taken from the side of 

 the tube. This is of course due to what we may term the 

 first stage of reversal, certain satellite lines being weakened 

 by absorption more than others, the phenomenon being 

 analogous to one which I mentioned some years ago, namely 

 that a hydrogen tube appears red when viewed from the side, 

 and bluish white when seen end-on (Phys. Zeitschrift, 1906, 

 p. 926). The time of exposure varied from 30 minutes to an 

 hour, which compares favourably with the time required with 

 large echelons. 



The last paper on the structure of the mercury lines which 

 I have been able to find is that of Lunelund (Annalen der 

 Physilc, xxxiv. p. 505, 1911), and as comparative tables of the 

 observations made by all previous observers are given in this 

 paper, I shall make no other references. From a comparison 

 of these tables with the photographs obtained with the 

 grating one cannot but help feeling grave doubts about 

 accepting the results yielded by the echelon or interference 

 plate until they have been checked up with a grating, except 

 in special cases. This is especially so in the cases where the 

 width of the group of satellite lines is greater than the dis- 

 tance between the spectra of adjacent orders, which is often 

 the case. 



It is customary to give the positions of the satellite lines 

 to the right and left of what is called the main line, in terms 

 of -4- and — fractions of an Angstrom Unit. Some am- 

 biguity results from this owing to the circumstance that 

 different observers do not always choose the same point for 

 the main line ; as for instance in cases where the main line 

 is a doublet, an observer working with an instrument in- 

 capable of resolving it would choose its centre of gravity as 

 his zero point, while an observer who resolved the line would 

 choose the stronger component. An example of this will be 

 found in the case of my results on the 4359 line. 



My own study of the lines was made at the end of the 

 summer, just before the closing up of my laboratory, with a 

 view of determining whether any new data of value could be 

 obtained. At the time I was unable to refer to any of the 

 previous papers on the subject, and consequently was unable 

 to make a further study of the discrepancies which were 

 found as soon as comparisons were made. Next summer it is 

 my plan to place the spectrograph in an underground tunnel, 

 and secure a larger grating if possible, for the results obtained 

 already indicate that but very little further improvement will 



