IN CONNEXION WITH THE SPECTRUM OF THE SUN. 
807 
time ago by Mr. Rutherfurd (and which I have not yet been able to equal*), was first 
divided (from 3900 to 5000) into portions corresponding each to a difference of wave- 
length of millim. by identifying in the print the more prominent of the Fraun- 
hofer lines mapped by Angstrom, and attaching the proper numbers to the consecutive 
divisions. The spaces between these larger divisions were then, by a similar use of 
Angstrom’s map, divided into tenths, each of these divisions corresponding consequently 
to a difference of wave-length of , J: - millim. The millimetres of Angstrom’s scale 
(equal to a difference of wave-length of millim. each) were found by dividing the 
I00 ~ j millim. divisions into ten equal parts. From this standard print the position of 
any new line is found by carefully observing (in a photograph of the spectrum of the 
metal confronted with the solar spectrum) the precise position of the new line with 
respect to the solar spectrum, and then finding the corresponding position in the printf . 
Although absolute accuracy cannot be claimed for the positions of lines determined in 
this way, I have nevertheless expressed the wave-lengths in the Tables in the usual scale 
(1-10 10 metre) in order to distinguish between lines which occur close together, of 
which there are several instances in the present Map. 
The lengths of the new lines are determined from special photographs (Plate 
LXXXIY.), to obtain wdiich a horizontal arc is employed, as explained in my last paper. 
Reversal of the new lines in the Solar Spectrum. 
With regard to the reversal or non-reversal in the solar spectrum of the new lines 
revealed by the photographs, I refrain for the present from making any positive state- 
ments, as I have not yet been able to obtain sufficiently good photographs of the 
metallic and solar spectra confronted. I may add, however, that lines are to be found 
in Mr. Rutherfurd’s photograph of the solar spectrum in the positions, within the 
limits of error, of nearly all the new lines ; but the close approach to coincidence between 
lines undoubtedly distinct, which I have observed in many cases in the course of con- 
structing the accompanying Map, convinces me that in future much greater precision 
will be necessary before assigning any line in the solar spectrum to a particular metal, 
and I therefore reserve the question of the reversal of the new lines till our knowledge 
of the solar spectrum is more complete. For a similar reason it is obvious that greater 
attention will have to be given to the precise character as well as to the position of 
each of the Fraunhofer lines, in the thickness of which I have already observed several 
anomalies. I may refer more particularly at present to the two H lines 3933 and 3968 
belonging to calcium, which are much thicker in all photographs of the solar spectrum 
than the longest calcium line of this region (4226'3), this latter being invariably thicker 
than the H lines in all photographs of the calcium spectrum, and remaining, moreover, 
visible in the spectrum of substances containing calcium in such small quantities as not 
to show any traces of the H lines. How far this and other similar variations between 
* Owing, among other causes, to constant vibrations incident to observations in London. 
f A photograph in which all the metals mapped are confronted on the same plate is a useful guide for 
checking the positions of the lines when found as above. 
MDCCCLXXIV. 5 Q 
