SECT. XL. 3. OCULAR SPECTRA. 615 



2. If you look for fome minutes ileadily 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, 

 reprefenting the frame-work, will appear to lie 

 acrofs the glafs panes : for thofe parts of the retina, 

 which were before Icafl 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 luminous parts of the window. 



3. M^ke 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 tad- 

 pole, as in plate 2, at Sec~l. III. 8. 3. ; look fteadily 

 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 tad- 

 pole 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 ex- 

 periment 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 experiment. 



4. On clofing the eyes afier viewing 'the black 

 fpot on the white paper, as in the foregoing expe- 

 riment, a red fpot is fecn of 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 more 

 expofed to the white paper, is capable of perceiv- 

 ing the red rays which penetrate the eyelids. If 



this. 



