Df Brewster on Vision ihrouglt Coloured Glasses: 1^3 
provided each of them transmits equal quantities of homogene- 
ous light ‘K 
Impressed with this opinion^ I was surprised to find, that vi- 
sion through a piece of blue glass became so painful to the eye, 
that it was not able to endure the impression for any length of 
time. In order to discover the cause of this unexpected effect, 
I examined with a prism the light of a candle transmitted through 
the blue glass, and found that it had the remarkable property of 
absorbing only the middle rays of the spectrum, viz. the Green^ 
Yellow and Orange^ and transmitting the Violet and the Red, 
The spectrum, therefore, consisted of two separate images, tjie 
one Red and the other Blue ; and hence the eye was not able to 
see distinctly by means of rays of such different refrangibilities. 
When it tried to adapt itself to the blue rays, it became inca- 
pable of converging the red ones ; and when it endeavoured to 
converge the red light to a focus, it lost the power of converging 
the blue' The effect, in short, was the same as if it had at- 
tempted, by its muscular power, to adjust itself to two different 
distances at the same time, and therefore it became completely 
exhausted with its fruitless efforts to obtain distinct vision. 
If the eye is adjusted, so as to see a luminous point through 
the blue glass, by means of blue light, the blue image of the 
point will be surrounded with a circle of red light, which is a 
section of the cone of red rays that the eye has not converged to 
a focus; and, in like manner, when the ej^e is adjusted to see 
the luminous point by the red light, the red image of the point 
is surrounded with a circle of blue light, which is a section of 
the cone of blue rays, while diverging from their focus within 
the vitreous humour. 
This striking example of the imperfection of vision through 
glasses of a compound colour, points out the principle upon 
which they should be selected. As the coloured glasses which 
are fitted for ordinary vision cannot be made to transmit homo- 
® Glasses which transmit only rays of one colour are very rare, and are of great 
use in many optical experfments. I have often combined plates of differently co- 
loured glass, so as to produce this effect completely. The only artificial glass which 
I have met with possessing this property in perfection, is one of a fine blue colour, 
which transmitted only the red rays of the spectrum. 
