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ocular SpeSlra of Light and Colours . j 2 1 
light than the other parts of the retina which were expofed to 
the more luminous parts of the window. 
3. Make with ink on white paper a very black fpot, about 
half an inch in diameter, with a tail about an inch in length, 
fo as to reprefent a tadpole look fteadiiy for a minute on this 
fpot, and, on moving the eye a little, the figure of the tadpole 
will be feen on the white part of the paper, which figure of 
'the tadpole will appear whiter or more luminous than the other 
parts of the white paper ; for the part of the retina on which 
the tadpole was delineated, is now more fenfible to light than 
the other parts of it, which were expofed to the white paper. 
This experiment is mentioned by Dr. Irwin, but is not by 
him afcribed to the true caufe, namely, the greater fenfibility 
of that part of the retina which has been expofed to the black 
fpot, than of the other parts which had received the white 
field of paper, which is put beyond a doubt by the next expe- 
riment. 
4. On doling the eyes after viewing the black fpot on the 
white paper, as in the foregoing experiment, a red fpot is feen 
bf the form of the black fpot : for that part of the retina, on 
which the black fpot was delineated, being now more fenfible 
to light than the other parts of it, which were expofed to the 
white paper, is capable of perceiving the red rays which pene- 
trate the eyelids, if this experiment be made by the light of 
a tallow candle, the fpot will be yellow inflead of red ; for 
tallow candles abound much with yellow light, which paffes 
in greater quantity and force through the eyelids than blue 
light ; hence the difficulty of diftingu idling blue and green by 
this kind of candle light. The colour of the fpe&rum may 
poffibly vary in the day light, according to the different colour 
of the meridian or the morning or evening light. 
Vol. LXXVL U u 
M. Be- 
