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Art. XVI I . — Ohservatlons on Vision throu^li Coloured Glasses, 
and on their applkatio7i to Telescopes, and to Microscopes of' 
great magnitude. By David Brewster,' Lli. D. F. R. S. L. 
& See. R. S. Edin. 
I AM not aware that any observations have hitherto been made 
Dn the subject of vision through coloured glasses. The astrono- 
mer has long been in the habit of using them to attenuate the 
light, and to obstruct the heat of the solar rays ; the painter 
occasionally employs them to give a warmer tint to his land- 
scape ; and, in cases where the human retina is extremely sen- 
sible to light, or where other parts of the eye are not capable of 
sustaining its strong impressions, coloured media have been adopt- 
ed, to reduce the incident rays to a proper degree of dilution. 
The colour generally selected for the relief of tender vision, 
has been a bluish or yellowish Green ; and the choice seems to 
have had no other foundation, than the vague analogy that the 
eye was best fitted to bear the impression of those rays which 
Nature had shed most abundantly over her works. Fashion 
lias, however, substituted a sort of Blue or Grey medium in 
^ place of green ; and, unless checked by the application of some 
principle, may soon carry us through all the colours of the spec- 
trum. 
When we consider light as consisting of several distinct rays, 
differing in refrangibility, and on this account creating imperfect 
vision, by their imperfect convergence on the retina, it is easy 
to understand how this imperfection may be removed, by look- 
ing through a medium which transmits only rays of a particular 
colour. In this point of view, every homogeneous colour should 
afford nearly the same relief ; and if we abstract tire diffe- 
rent heating powers of the coloured rays, which, in ordinary 
lights, can have no influence, it is difficult to discover any rea- 
son why one coloured medium should be preferred to another, 
Read before Ihc Royal Society of Edinburgh, November 19. 182 L 
-|’ Dr HerGchel has publiohcd in the Phil. Trans, for 1800, p. 255. an account 
of oomc very interesting cxperimenlG on the power of coloured glasses to intercept 
ddicrent rays, particularly the red, or those which licat most powerfully. 
