MOTIONS OF Sect. III. i. i 



SECT. III. 



THE MOTIONS OF THE RETINA DEMONSTRATED BY EXPERI- 

 MENTS. 



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

 retina. III. The aBiviiy of the retina in v'fion. I. Rays of 

 light hove no momentum. 2. Objects long viewed become fainter. 



3. Sped r a of black objects become luminous. 4. Varying fpeBra 

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



Motions of the organs of fenfe confiitute ideas. 1 . Light from 

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

 tery. 2. Ideas in fieep mifiaken for perceptions. 3. Ideas of im- 

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

 organ of fenfe is defiroyed, the ideas belonging to that fenfe perifh. 

 V. Analog") between mufcular motions and fenfual motions, or 

 ideas. 1. %hey are hot;: originally excited by irritations. 2. And 

 officiated together in the fame manner, 3. Both act in nearly the 

 fame times. 4. Are alike firengthened or fatigued by exercife. 

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

 ed by comprejfion. 7. Are alike liable to parahfts. 8. To con- 

 njufion. 9. To the influence of old age. — VI. ObjeBions anfwered. 

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

 nal object s. 3. Of the imagined fen fation in an amputated limb. 



4. Alfiracl ideas. — VII. What are ideas, if they are not ani- 

 mal motions ? 



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

 ranged into natural claiTes and orders, it is neceffary to fmooth 

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

 fome obstacles which thwart our paffage. I. To demonftrate 

 that the retina and other immediate organs of fenfe pofTefs a 

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

 according to the fifth and feventh of the preceding afTertions, 

 claims our firft attention. 



Animal motions are diftinguiihed from the communicated 

 motions, mentioned in the firft fection, as they have no me- 

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

 &in of a hcrfe ftiall induce him to move a load of hay. They 

 diircr from the gravitating motions there mentioned as they arc 

 with equal facility in all directions, and they differ from 

 e chemical clafs of motions, becaufe no apparent decompofi- 

 - o;- new combinations are produced in the moving ma- 

 is, 



Hence, 



