448 OCULAR SPECTRA. SECT. XL. 3. i. 



cited into their due a&ionby thelefs ftimulus of common food 

 alone ; of which the immediate confequence is indigeftion and 

 hypochondriacifm. 



III. OF SPECTRA FROM EXCESS OF SENSIBILITY. 



The retina is more eqfily excited into aEiion by greater irritation after 

 having been lately fubjeuled to lefs. 



1 . IF the eyes are clofed, and covered perfeftly with a hat, 

 for a minute or two, in a bright day ; on removing the hat a 

 red or crimfon light is feen through the eyelids. In this exper- 

 iment the retina, after being fome time kept in the dark, becomes 

 fo fenfible to a fmall quantity of light, as to perceive diftindlly 

 the greater quantity of red rays than of others which pafs through 

 the eyelids. A (imilar coloured light is feen to pafs through the 

 edges of the fingers, when the open hand is oppofed to the flame 

 of a candle. 



2. If you look for fome minutes fteadily on a window in the 

 beginning of the evening twilight, or in a dark day, and then 

 move your eyes a little, fo that thofe parts of the retina, on 

 which the dark frame-work of the window was delineated, may 

 now fall on the glafs part of it, many luminous lines, repre- 

 fenting the frame- work, will appear to lie acrofs the glafs panes: 

 for thofe parts of the retina, which were before lead ftimulated, 

 by the dark frame- work, are now more fenfible to light than the 

 other parts of the retina which were expofed to the more lu- 

 minous 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, as in plate 2, at Sefc. III. 8. 3 ; look 

 (leadily for a minute on this fpot, and, on moving the eye a lit- 

 tle, 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 ex- 

 pofed 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 re- 

 ceived the white field of paper, which is put beyond a doubt by 

 the next experiment. 



4. On clofing the eyes after viewing the black fpot on the 

 white paper, as in the foregoing experiment, a red fpot is feen 



of 



