SECT. III. 3. MOTIONS OF THE RETINA. , 21 



on, or a chemical tinge of coloured light, the per- 

 ception would every minute become ftronger and 

 ftronger whereas in this experiment it becomes 

 every inftant weaker and weaker. The fame cir- 

 cumflance obtains in the continued application of 

 found, or of fapid bodies, or of odorous ones, or 

 of tangible ones s to their adapted organs of icnfe. 



Thus when a circular coin, as a {hilling, is preffed 

 on the palm of the hand, the fenfe of touch is me- 

 chanically compreffed ; but it is the flimulus of this 

 preflure that excites the organ of touch into ani- 

 mal action, which conftitutes the perception of 

 hardnefs and of figure : for in feme minutes the 

 perception ceafes, though the mechanical prefTure 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 ftedfaftly for a minute on the center 

 of this fpot, and on moving the eye a little, the 

 figure of the tadpole \vill be feen on the white part 

 of the paper j which figure of the tadpole will ap- 

 pear more luminous than the other part of the white 

 paper ; which can only be explained by fuppofing 

 a 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 mecha- 

 nical impreffion or chemical combination of light 

 with the retina. 



4. When any one turns round rapidly, till he be- 

 comes dizzy, and falls upon the ground, the fpec- 

 tra of the ambient objects continue to prefent them- 

 felves in rotation, and he feems to behold the ob- 

 jects ftillin motion. Now if thefe fpectra were im- 

 preflions on a paffive organ, they either muft conti- 

 nue as they were received laft, or not continue at 

 ill, 



5- Placq 



