240 Dr. Darwin’s Experiments on the 
the fcin filiations of the ftar Sirius to be fometimes coloured, 
thefe were probably the diredt fpedtrum of the blue (ky on the 
farts of the retina fatigued by the white light of the ftar* 
(Eflays Phyfical and Literary, p. 81. V. 2.) 
When a dir eft fpedtrum is thrown on colours darker than 
Itfelf, it mixes with them; as the yellow fpedtrum of the 
fetting fun, thrown on the green grafs, becomes a greener- 
yellow. But when a diredt fpedtrum is thrown on colours 
brighter than itfelf, it becomes inftantly changed into the 
reverfe fpedtrum, which mixes with thofe brighter co- 
lours. So the yellow fpedtrum of the fetting fun thrown on 
the luminous Iky becomes blue, and changes with the colour 
or brightnefs of the clouds on which it appears. But the re- 
verfe fpedtrum mixes with every kind of colour on which it is 
thrown, whether brighter than itfelf or not : thus the reverfe 
Ijpedtrum, obtained by viewing a piece of yellow filk, when 
thrown on white paper was a lucid blue green ; when thrown 
on black Turkey leather becomes a deep violet, And the fpec- 
trum of blue filk, thrown on white paper, was a light yel- 
low; on black filk was an obfcure orange; and the blue fpec- 
trum, obtained frGm orange-coloured filk, thrown on yellow, 
became a green. 
In thefe cafes theretina is thrown into activity or fenfation 
by the ftimulus of external colours, at the fame time that it 
continues the adtivity or fenfation which forms the fpedtra ; in 
the fame manner as the prifmatic colours, painted on a whirl- 
ing top, are feen to mix together. When thefe colours of ex- 
ternal objects are brighter than the diredt fpedtrum which is 
thrown upon them, they change it into the reverfe fpedtrum, 
like the admiffion of external light on a diredt fpedtrum, as 
explain ed above. When they are darker than the diredt fpec- 
trura 
