12 MOTIONS OF SECT. III. 3. 3. 



becomes fainter, till at length, it entirely vanifhes, though the 

 eye is kept uniformly and fteadily upon it. Now if the change 

 or motion of the retina was a mechanical impreflion, or a chem- 

 ical tinge of coloured light, the perception would every minute 

 become ftronger and ftronger ; whereas in this experiment it 

 becomes every inftant weaker and weaker. The fame circum- 

 ftance obtains in the'continued application of found, or of fapid 

 bodies, or of odorous ones, or of tangible ones, to their adapted 

 organs of fenfe. 



Thus when a circular coin, as a (hilling, is prefled on the 

 palm of the hand, the fenfe of touch is mechanically comprefE- 

 ed ; but it is the ftimulus of this preflure that excites the organ 

 of touch into animal a6Hon, which conftitutes the perception of 

 hardnefs and of figure : for in fome minutes the perception 

 ceafes, though the mechanical preflure of the object remains. 



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 refemble a tadpole, as in Plate II. ; look fleadfaftly for a 

 minute on the centre of 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 more lumi- 

 nous than the other part of the white paper ; which can only 

 be explained by fuppofing that part of the retina, on which the 

 tadpole was delineated, to have become more fenfible to light 

 than the other parts of it, which were expofed to the white 

 paper ; and not from any idea of mechanical impreflion or 

 chemical combination of light with the retina. 



4. When any one turns round rapidly, till he becomes dizzy, 

 and falls upon the ground, the fpedtra of the ambient objects 

 continue to prefent themfelves in rotation, and he feems to be- 

 hold the objects ftill in motion. Now if thefe fpeftra were im- 

 preflions on a paflive organ, they either mufl continue as they 

 were received laft, or not continue at all. 



5. Place a piece of red filk about an inch in diameter on a 

 fheet of white paper in a ftrong light, as in Plate I. ; look ftead- 

 ily upon it from the diftance of about half a yard for a minute ; 

 then clofmg your eye-lids, cover them with your hands and 

 handkerchiei, and a green fpe&rum will be feen in your eyes 

 refembling in form the piece of red filk. After fome feconds 

 of time the fpeftrum will difappear, and in a few more feconds 

 will reappear ; and thus alternately three or four times, if the 

 experiment be well made, till at length it vanifhes entirely. 



6. Place on a fheet of white paper a circular piece of blue 

 {ilk, about four inches in diameter, in the funfhine ; cover the 

 centre of this with z circular piece of yellow filk, about three 



inches 



