Mr Smith on tJie Jluctuating Sensibilit/y of the Retina. 97 
more remarkable, when I closed my eyes, and covered them 
with several folds of black cloth to exclude the light, the spec- 
trum and halo appeared more brilliant than when the eyes were 
opened upon a white wall. 
This experiment has been often repeated by myself and o-. 
thers, with every precaution to keep out the light, but invariably 
the spectrum and halo have been found brighter the more per- 
fectly the light was excluded from the eyes. 
When the degree of light proceeding from the more luminous 
body or surface is not so great as to dazzle the eyes, I have 
found the state of the sensibility, after viewing the object, to be 
exactly the reverse of what it was at the time the eyes were fix- 
ed upon it. But when the eyes bore with difficulty the quanti- 
ty of light reflected or emitted from the more luminous object, 
the same regular reverse accidental colours were not obtained. 
One thing, however, is remarkable, that the colours of the spec- 
trum and halo are, in such cases, as far as I have found, exact- 
ly the reverse of one another. Several experiments exemplify- 
ing this, though somewhat imperfectly, are on record. I shall 
subjoin only one. 
Experiment 8. — This experiment was made with my eyes 
shut and closely covered up from the light ; after I had looked 
steadily at the centre of the oiled paper, described in experi- 
ment 5., placed between the sun and my eyes, till the halo be- 
came viyid. The whole experiment took up several minutes, 
and the numbers refer to the observations made at intervals du- 
ring that time. 
1. Spectrum yellov/, with a reddish border : bluish purple 
halo. % Spectrum disappeared. 3. Yellow spectrum : purple 
halo. 4. Disappeared. 5. Red spectrum, surrounded by dif- 
fused greenness. 6. Disappeared. 7. Deeper red spectrum : 
light green halo, 8. Disappeared ; but suddenly 9. Purple 
spectrum: yellow halo. 10. Disappeared, 11. Darker purple 
spectrum, surrounded by diffused light-yellow. 12. Disappear- 
ed. ]3. Broken down spectrum, which became gradually 
fainter, till it finally disappeared. 
Having thus given a sketch of the principal facts which I 
have observed upon the state of the sensibility of the retina at 
the time of, and after viewing equally and unequally illumh 
VOL. in. NO. 5. JULY 1820. 
G 
