On Solar Radiation. 113 



error will arise from the glasses not applying with sufficient 

 accuracy to the faces of prism. No appreciable error need be 

 apprehended from the refractive power of the glasses, pro- 

 vided they be of the kind used for making the mirrors of sex- 

 tants ; and should any notable imperfection exist, it will 

 manifest itself by the separation of the images reflected from 

 the two surfaces of the defective glass. 



With reference to the other source of error, I may refer to 

 the results of actual observation, to shew within what narrow 

 limits it may be confined ; and in doing so, I have to acknow- 

 ledge the kindness of Mr William Nicol, Mr John Adie, and 

 Mr Alexander Bryson, to whom I am indebted for the use of 

 most of the prisms whose angles have been determined by 

 this method. 



The angle of a flint-glass prism, belonging to Mr Adie, was 

 taken five times, and the greatest and least result differed by 

 only 50", the object observed being a street lamp nearly 400 

 feet distant. The angle of the same prism was then deter- 

 mined by a double observation on the turrets of Trinity Chapel, 

 Deanbridge, at a distance of about J of a mile, and the result 

 differed from the mean of several accurate experiments made 

 some years ago by Mr Adie, by 30". A plate-glass prism, ex- 

 amined in the same way, gave a result difi^ering from Mr Adie's 

 by 50", and the refractive indices of these prisms differed from 

 Mr Adie's" results only in the fourth place of decimals. A simi- 

 lar agreement exists between many other observations I have 

 made on different substances and the results obtained by for- 

 mer observers, so that I feel quite satisfied with the practical 

 efficiency of the process. 



Edinburgh, 4 Duke Street, lOih June 1843. 



On Solar Badiation, 



The experiments mentioned in the text (page 215), refer- 

 red to a curious inquiry which has occupied my attention for 

 some years, namely, the loss of force which the sun's rays ex- 

 perience in passing through the earth's atmosphere. It might 



VOL. XXXVI. NO. LXXI. JAN. 1844 H 



