We need not inquire what is the absorptive power of the thermo- j unction, provided that we are justified in assuming that lamp-blacked surfaces absorb the radiation from the hot tube as freely as that from the sun, or that the constants in these expressions (1) and (2) for the radiation may be taken to be equal.
On balancing, these expressions must be equal, and therefore
(100) 2 '
(T \ 4 =^ ) may be neglected, hence we have finally
04 iw
ird 2 sin 2 p p
V 10000 YljLl v 7TC? 2 v ^sin 2 p
0-13806 4 /7[ J~s^~o ' v PQ'
or
pq sin- p
The mean value of -^ - is [1-30413]. v sin/a
Therefore 0=1- 3041 3 . ? / JL .
After a series of observations had been made, the furnace and tube were raised so that the radiation of the latter then passed into the aperture (A), on which the sunlight had previously fallen, while the beam of sunlight was now directed so as to be upon (B), and in this position a second series of observations was taken. The geometrical mean of the result of the two groups gives the Effective Temperature of the Sun, the effect of any difference in the sensitiveness of the thermo-j unctions disappearing in the geometrical mean.
Observations were made in the manner described above on August 19th and September 30th, 1901, and reduced by means of equation (4), as exhibited in the following tables. In these the successive columns contain (1) the local mean time, (2) the value of /? as read on the micrometer head, (3) the absolute temperature of the tube in the furnace, (4) the sun's altitude, (5) the percentage of the sun's radia- tion transmitted through the earth's atmosphere, (6) the angle of incidence on the heliostat mirror, (7) the percentage reflected from the surface of the mirror, (8) the corresponding value of deduced from equation (4). Of these (5) and (6) and (7) were determined as in Wilson and Gray's memoir referred to above.
