[ 225 ] 
XIV. The Bakerian Lecture. — On the Transparency of the Atmosphere and the 
Law of Extinction of the Solar Rays in passing through it. By James D. Forbes, 
Esq., F.R.S., Sec. R.S. Ed., Corresponding Member of the Royal Institute of 
France, and Professor of Natural Philosophy in the University of Edinburgh. 
Received May 20, — Read May 26, 1842. 
Contents. § I. Qualities of Rays. §11. History of the Inquiry . § III. On the 
Mass of Atmospheric Air traversed by Rays with varying obliquities. § IV. Ac- 
count of the following Observations : — Comparison of Actinometers — Reduc- 
tion to Intervals of One Minute. § V. Analysis of the Observations of the 25 th 
of September, 1832. § VI. Concerning the Law of Extinction. § VII. Other 
Observations in 1832. § VIII. Observations in 1841. § IX. Conclusions. 
1. THE experiments which will chiefly be analysed in this paper were made nearly 
ten years since. I have been deterred from drawing the conclusions from them 
which they warrant, partly by the great labour of the necessary reductions, and 
partly by peculiar and inherent difficulties which this intricate subject presents. For 
the computations I am much indebted to the perseverance and care of Mr. John 
Broun (now magnetical assistant to Sir Thomas Brisbane), who has made most of 
them under iny eye, and also to Mr. James Stark. For much which yet remains 
obscure and uncertain in my conclusions, I anticipate the indulgence of those best 
acquainted with the uncertainties under which the subject of absorption, whether of 
light or heat, is still veiled, and with the little advance which has been made in the 
particular branch which we have to consider, namely the law of extinction of solar 
light and heat in the atmosphere. 
2. Permanently enclosed as we are within an imperfectly transparent shell which 
separates us from the realms of space, a knowledge of the various properties of the 
atmosphere, especially as regards light and heat, is peculiarly important in the reso- 
lution of many cosmical problems. We cannot at will place ourselves, as it were, in 
a point in space, until we can eliminate the effects of this transmission. Hence the 
great importance of the subject of astronomical refractions, one nearly allied to 
the present, and which has exercised, from the time of Newton to that of Ivory, the 
happiest skill of some of the most eminent analysts and natural philosophers. The 
difficulties of the doctrine of astronomical refractions it has in common with that ot 
the extinction of light, and to these are superadded many more, owing to the incom- 
parably inferior methods which we have of measuring both light and heat compared 
to the measure of angular quantities. 
MDCCCXLII. 2 G 
