ON COLOUR PHOTOMETRY. 
427 
against the slit in the focus of the spectrum, it was easy to see when the position of the 
slit corresponded with that of any line ; the reading of the scale on the slide of course 
gave the position of the line. The positions of these lines being obtained, wave-lengths 
for different parts of the spectrum could be interpolated. When sources of light other 
than the sun or the electric arc had to be employed, a magnesium wire on which a 
small quantity of chloride of lithium in powder had been placed was burnt in front of 
the slit. This gave clearly the lines of magnesium, sodium (D), lithium (red), with 
sometimes a glimpse of the blue lithium line. The position in the blue could be fixed 
by means of the lines in the base of a candle-flame. But it was rarely necessary to 
resort to these, since the scale of the spectroscope was sufficiently well known to enable 
any position to be calculated when one or two lines were fixed. 
The following table shows the wave-lengths of the scale used in all subsequent 
tables :— 
Table of Wave-lengths. 
Scale No. 
x 
Scale No. 
X 
43 = 
699 
52 
— 
487 
44 = 
662 
53 
-_-Z 
474 
45 = 
629 
54 
= 
464 
46 = 
601 
55 
— 
454 
47 = 
577 
56 
— 
445 
48 = 
557 
57 
— 
436 
49 = 
538 
58 
— 
428 
50 = 
519 
59 
420 
51 = 
502 
60 
= 
412 
§ IV. The Source of Light. 
It is unnecessary to call attention to the great importance of selecting a suitable 
source of light. For the sake of convenience and comfort the light should be bright, 
for accuracy of reading it should be steady, for comparison it should be readily obtain¬ 
able, and at the same time have a constant total illuminating value. The natural 
inclination is to turn to sunlight for such a source ; but it is neither steady from 
minute to minute, except on rare occasions, nor in this climate of ours readily obtain¬ 
able. The flame of a candle or of an oil lamp is too feeble, and the same applies to the 
light from an electrical glow lamp, except under favourable circumstances. Such being 
the case, we turned to the electric arc light, and found, as we have found in so many 
of our experiments, the results of which we have had the honour of laying before the 
Royal Society, that with proper management it gives a steady light and has a constant 
temperature, as our many measurements of the spectrum illumination showed. We used 
a hand lamp with inclined carbons (fig. 2), which we had for convenience sake slightly 
altered from Sautter Lemonier’s pattern. The top carbon, which was the positive pole, 
3 t 2 
