ARC SPECTRUM OP ELECTROLYTIC IRON. 
991 
as well as that at Kensington, but the evidence in the second column rests solely on 
the new unpublished map which is in course of construction at Kensington. 
Comparison ivitli Thalen’s Eye Observations. 
The position of all lines, for which the corrections differ considerably from the 
mean, have been very carefully determined from the scale of wave-lengths attached 
to Howland’s maps, so that a few slight corrections to some of Thalen’s measures 
appear to be indicated. Thus in the region between 4600 and 4700, the mean 
difference of wave-length on the two scales is -f- 1 'O. On the photographs a strong 
line occurs coincident with a Fraunhofer line at 4667’6. Thalen gives the wave¬ 
length of this line as 4665'5. The difference is therefore 2T instead of about I'O. 
It seems probable that a misprint has occurred, and that 4665'5 should be 4666'5, 
in which case the difference would be 1‘1. 
Thalen’s estimation of the intensities of the lines generally agrees with the 
intensity in the photographs. The lines showing the most striking differences in 
this respect are 4432‘8, 4433'4, and 4434‘0. They are given the intensities 4, 3, 4 
by Thalen, in the photographs, however, their intensities are 6, 5, 6. 
Some of the single lines given by Thalen have been found to be double, and a few 
which he suspected were double have been proved to be so. These cases are shown 
in the tables. 
In all regions, except that contained between K and G, Thalen observed more 
lines than are found in the photographs. The number of lines observed in each case, 
in all the regions contained in the appended tables, are as follows :— 
Regions 
compared. 
Lines mapped from the 
photographs. 
Lines observed by 
Thalen. 
\4000-G 
257 
194 
G-F 
254 
266 
r-& 
120 
188 
5-D 
187 
213 
D-C 
55 
147 
Total . 873 
Total . 1008 
More lines would doubtless have been obtained, in the region from D to C, if 
a longer exposure had not been prevented by a want of electrolytic iron. 
In addition to the photographic lines which appear to be due to impurities in the 
electrolytic iron, several lines common to Thalen and the photographs, and some 
given by Thalen, but not found in the photographs, most probably require to be 
eliminated from the spectrum of iron proper. The lines having their origin in 
