MOTIONS OF SECT. III. i. i. 



SECT. HI. 



.THE MOTIONS OF THE RETINA DEMONSTRATED BY EXPERI- 

 MENTS. 



I. Of animal motions and of ideas. II. The fibrous Jlruflure of the 

 retina. III. The activity of the retina in vifan. i . Rays of 

 light have no momentum 2. Objecls long viewed become fainter. 



3. SpecJra of black objects become luminous. 4, Varying fpeclra 

 from gyration. 5. From long infpeclion of various colours IV. 



Motions of the organs offenfe conftitute ideas. I. Light from 

 prejfing the eye-ball, and found from the pulfation of the carotid ar- 

 tery. 2. Ideas in Jleep miflaken for perceptions. 3. Ideas of im- 

 agination produce pain andficknefe like fen fations. 4. When the 

 organ offenfe is deftroyed> the ideas belonging to that fenfe perijh. 

 V. Analogy between mufcular motions and fenfual motions, or 

 ideas. I. They are both originally excited by irritations. 2. And 

 officiated together in the fame manner. 3. Both acJ in nearly the 

 fame times. 4. Are alike Jlrengthened or fatigued by excercife 9 

 5. Are alike painful from inflammation. 6. Are alike benumb- 

 ed by compreffion. 7. Are alike liable to paralyfts. 8. To con- 

 vulfion. 9 . 21? the influence of old age VI (ibjeclions anfwered 

 I. Why we cannot invent new ideas. 2. If ideas refemble exter- 

 nal objecls. 3 . Of the imagined fenfation in an amputated limb. 



4. Abjlral ideas. VII. What are ideas y if they are not ani- 

 mal motions ? 



BEFORE the great variety of animal motions can be duly ar- 

 ranged into natural claifes and orders, it is neceflary to fmooth 

 the way to this yet unconquered field of fcience, by removing 

 fome obftacles which thwart our paflage. I. To demonftrate 

 that the retina and other immediate organs of fenfe poflHs a 

 power of motion, and that thefe motions conftitute our ideas, 

 according to the fifth and feventh of the preceding aflertions, 

 claims our firlt attention. 



Animal motions are diflinguifhed from the communicated 

 motions, mentioned in the firit feclion, as they have no me- 

 chanical proportion to their caufe ; for the goad of a fpur on the 

 Ikin of a horfe ihall induce him to move a load of hay. They 

 differ from the gravitating motions there mentioned as they are 

 exerted with equal facility in all directions, and they differ from 

 the chemical clafs of motions, becauie no apparent decompofi- 

 tions or new combinations are produced in the moving ma- 

 terials. 



Hence, 



