Mi- Ritchie on Leslie 'a Photometer. 328 



cations of the instrument under so many obstructing causes. 

 Mr Leslie, with his usual ingenuity, has endeavoured to re- 

 move some of these disturbing causes, particularly the reflec- 

 tion of light from the surface of the earth ; but, though this 

 may be removed, the others remain in full force, and exert a 

 powerful influence. Without making, therefore, a proper al- 

 lowance for these obstructing causes, the indications of the 

 photometer do not afford even an approximation to the rela- 

 tive intensity of the solar rays at different periods and in dif- 

 ferent situations. 



It has lately been made a question, whether the photometer 

 is acted upon by mere heat, unaccompanied with light. Both 

 experiment and reasoning concur to prove, that mere heat can 

 have no influence whatever upon it, unless that heat move 

 with a velocity sufficient to permeate the glass case by which it 

 is surrounded.* If the instrument be placed opposite a ball 

 of iron heated almost to redness, no effect whatever will be 

 produced ;-f- but, if the temperature of the ball be raised so as 

 to shine in the dark with a dusky red colour, the fluid in the 

 stem of the black ball will sink a considerable number of de- 

 grees. If the temperature of the ball be raised still higher, it 

 will produce a greater effect upon the instrument than the 

 flame of the finest oil-gas, though the one possesses a much 

 greater illuminating power than the other. 



* The opinion here expressed by Mr Ritchie, is in direct opposition to 

 the experimental results of Delaroche and Berard, (recently confirmed 

 by Dr Turner and Dr Christison,) which have been almost universally ad- 

 mitted by philosophers. The Rev. Baden Powell, F. R. S. has still more 

 recently found, that the heat of luminous bodies, when intercepted by a 

 pane of glass, is separated into two portions, one of which is absorbed by 

 the screen, and the other transmitted, and that these two portions differ in 

 their properties, the heat absorbed being always equally absorbable by black 

 and white surfaces, while the heat transmitted is more easily absorbed by 

 black than white surfaces. — Ed. 



| Dr Turner and Dr Christison, have demonstrated by direct experi- 

 ments, which we, and many others have seen, that Mr Leslie's photome- 

 ter " is powerfully affected by heal," when placed " before a ball of iron 

 heated, so as not to be luminous, or even before a vessel of boiling water." 

 As Mr Ritchie has found the very reverse of this to be the case, we 

 may conclude, that an instrument which, in such hands, gives such op- 

 posite results, cannot deserve much confidence. — En. 



