MAPPING THE LEAST REFRANGIBLE ENG OF THE SOLAR SPECTRUM. 661 
photographs of the sun which were taken at Kew. Thanks to Mr. Whipple and his 
assistants, the arduous task of reading the measurements was materially lightened, 
which otherwise must have been protracted over a longer time than they have been. 
The instrument employed measures accurately to the y'ooo °f an i nc b and by estima¬ 
tion to half that quantity ; but I may say that this delicacy is rather too great for the 
photographs, and I can only trust my readings to of an inch. The magnifying 
power of the instrument was slightly too great for some of the finer and also for some 
of the less transparent absorption lines, and in the first measurements some escaped 
detection. By carefully going over the whole photographs with a less magnifying 
power the missing lines have been inserted with very fair accuracy; since most of them 
were situated close to lines which had been previously measured with the higher 
magnifying power. I believe that the accuracy of position of these inserted lines is 
but little, if anything, inferior to the remainder of the lines. After the whole of the 
photographs had been measured they were all reduced to a common scale by taking 
fiducial lines and interpolating in the usual manner. Except for a few of the last 
lines shown near wave length 9700, more than one photograph was measured, and 
the reason why only one photograph was used for this region was that that one 
employed had far better definition than the others, though all showed the presence of 
the absorption phenomena. 
In regard to the accuracy of reading I cannot do better than quote the measure¬ 
ments of the A group, to show the accordance between two sets of readings. 
Photo. I. 
Photo. II. 
Reduced to scale. 
A 0 
2778-0 
2778-0 
100-0 
A 0 
2824-0 
2825-0 
131-0 
A 0 
2826-5 
2827-0 
132-5 
A! 
2838-0 
2838-0 
142-0 
Ao 
2846-0 
2845-0 
147-0 
A 3 
2853-0 
2853-5 
152-0 
A 
2861-0 
2861-5 
157-0 
a b 
2870-0 
2869-5 
163-5 
Ag 
2880-0 
2879-5 
170-0 
a 7 
2890-0 
2888-5 
177-0 
A 8 
2900-0 
2899-5 
184-0 
A 
2910-0 
2910-0 
190-5 
The scale of the original map is 40 times the original measurement, 1 inch being 
represented by 40 inches.* The size of the plates used was 6 inches by 2^ inches 
wide, and from A to 10,750 occupies nearly that length. 
The slit of the collimator was tolerably narrow, but not quite so closed as is usually 
the case when photographs of the more refrangible end are taken. Thanks to the 
* For publication the scale has been reduced to 20 times the original measurement. 
MDCCCLXXX, 4 Q 
