56 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
intensity, and in some cases obliterates them. Indeed some coloured papers refuse 
to exhibit any thermographic impress whatever. 
138. Of the existence of a much more remote spot I have hardly a doubt; but it is 
very difficult to obtain a sight of it. Its place is at or about — 55, as measured on 
the first occasion, when it appeared with sufficient distinctness to be set down as a 
positive observation, at the same time that the spot & was placed by a similar measure 
at — 45*2. On another occasion (March 8), the sun being superbly clear, four mea- 
sures, considered at the time quite satisfactory, were obtained of it, the mean of 
which was — 63* 1 ; but this requires a correction for the position of the prism, which 
on this occasion was somewhat different from that which it had in previous experi- 
ments, it having been adjusted so as to throw the spectrum higher on the frame than 
before, for the sake of allowing plenty of room for the exhibition of the phenomena 
in question. But as on this occasion very numerous measures were also taken of the 
other spots, there are fortunately abundant data for calculating and applying the 
correction in question. I therefore set down all the measures so taken in the fol- 
lowing Table. 
Coord, of spot 
a 
|3 
y 
8 
€ 
- 23-0 
21*5 
19*0 
19-0 
- 28-0 
31*5 
31-8 
29-0 
29- 5 
30- 2 
33-0 
- 38-5 
41-0 
41-5 
38- 3 
39- 5 
40- 0 
41- 5 
41-0 
40-0 
- 48-4 
46-4 
49-0 
49- 5 
50- 0 
50*4 
50-5 
49- 0 
48-8 
50- 0 
51- 0 
51- 0 
50-2 
50-0 
52- 0 
- 62-7 
62*7 
63- 0 
64- 0 
Means ...... 
- 20-6 
- 30-4 
- 40-2 
- 49-7 
- 63-1 
If these numbers be now all reduced in the proportion of 402 to 35 - 7> so as to bring 
the spot y, which is the best-defined of any, to correspondence with the former re- 
sults, we have for the successive coordinates as follows : 
a 
P 
y 
8 
6 
- 18*2 
- 27-0 
- 35*7 
- 44-1 
- 56-0 
139. Whether these measures be considered or not as definitively establishing the 
existence of the spot s, the equality of distribution of them all along the axis of the 
spectrum is not a little remarkable, and would at first sight (from the analogy of the 
absorptive action of nitrous gas on the luminous rays) seem to point rather to a 
