I. On the Action of Transparent Bodies upon the differently 
coloured Rays of Light. By Daviv Brewster, LL. D. 
F. B.S. Lonv. & Epy. & F. A. S. Ep. 
(Read 5th June 1815.) 
‘ROM the intimate connexion of the present subject with 
the improvement of the Achromatic Telescope, it must 
be admitted to be one of the most important in Optics ; while, 
from the minuteness of the effects which are to be observed 
and compared, it is unquestionably one of the most difficult. 
From this cause very little progress has been made in the in- 
vestigation. The irrationality of the coloured spaces, in pris- 
matic spectra, formed by different substances, has not even 
been mentioned in any of our elementary treatises on Natural 
Philosophy, and there are some philosophers who have scru- 
pled to receive it as a truth established in Physics. 
In order to render this subject sufficiently intelligible, let us 
suppose that a ray of light is transmitted successively through 
two prisms, one of rock-crystal, and the other of flint-glass, ha- 
ving the same refracting angles. The rock-crystal will be 
found to bend the ray of light more from its primitive direc- 
tion than the prism of flint-glass. The former is therefore said 
_ Vox. VIII. P. I. A to 
