IN CONNEXION WITH THE SPECTRUM OF THE SUN. 
565 
Tlie drawing of section W.L. 390-400 millionths millim., exhibited with the pre¬ 
liminary note before referred to, was utilised for the first revision of the refraction 
map, for which purpose it was sufficiently good, although not good enough for copying 
finally, having been produced, as before mentioned, from a photographic enlargement 
of this region of the spectrum. 
The method adopted in drawing the final refraction map was the following. A 
millimetre scale of the required length having been drawn near the top of a sheet of 
paper prepared for the reception of the map, the fundamental lines were first entered 
in their correct positions in pencil upon it, and afterwards made of their correct 
widths and intensities, as seen in the standard photograph, by means of a drawing 
pen and Indian ink. The width of these fundamental lines, where sufficiently great, 
was found by actual measurements on the curve. The spaces between these funda¬ 
mental lines being too large on the scale adopted for interpolating the intermediate 
lines accurately by eye, their places in the photograph were found and their corre¬ 
sponding wave-lengths determined by means of the curve. 
The lines obtained in this manner by the aid of the interpolation curves were 
entered in the map in the same manner as before. 
A skeleton map is thus obtained, in which no greater extent is left blank than 
8 or 10 millims.—a distance sufficiently small, considering the shape of the curve, to 
allow of the interpolation of the intermediate lines by eye. The standard photograph, 
placed under a microscope of low magnifying power, was accordingly made to furnish 
the details for the remaining lines, which were interpolated with the greatest possible 
care, the interpolation position being checked from time to time by actual measure¬ 
ments of the wave-lengths of the selected lines on the curve. 
In all cases the agreement was as close as could be desired. 
In order to save the cost of engraving, a third map was constructed from the finally 
revised copy on twelve times the scale of Angstrom’s. This enlarged map was photo¬ 
graphed down to the original scale adopted for this work, viz., four times the scale 
o 
of Angstrom. For this enlarged map I was greatly indebted tc Lance-Corporal 
Murray, II, E., who made the drawing with great skill and care. 
y. Determination of the Wave-lengths of the Solar Lines. 
I have no means for determining wave-lengths absolutely, but the accurate measure¬ 
ment of the wave-length of the chief lines in the ultra-violet portion of the solar 
spectrum by Cornu rendered it unnecessary to attempt any absolute measurements 
for the construction of the present map. The last degree of accuracy in the determi¬ 
nation of wave-lengths is also, I think, having regard to the objects I have had in view, 
of less importance than good maps showing the details of the spectrum. The idea was 
at first entertained of simply copying Cornu’s map on the scale now adopted, and 
interpolating new lines by eye ; but for two reasons this idea had to be abandoned. 
