SECT. III. i. MOTIONS OF THE RETINA. 



SECT. III. 



THE MOTIONS OF THE RETINA DEMONSTRATED BY 

 EXPERIMENTS. 



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

 ftruclure of the retina. III. The aftivity of the re- 

 tina in vifion. i . Rays of light have no momentum. 

 2. Objects long viewed become fainter. 3* Spectra of 

 black objecls become luminous. 4. Varying fpeclra 

 from gyration. 5. From long injpedion of various 

 colours. IV. Motions of the organs of fenfe conftitute 

 ideas, i. Light from prejjing the eye-ball, and found 

 from the pulfation of the carotid artery. 2. Ideas in 

 Jleep mif taken for perceptions. 3. Ideas of imaginati- 

 on produce pain andjicknefs like fenfations. 4. When 

 the organ of fenfe is deftroyed, the ideas belonging to 

 that fenfe per ifh. V. Analogy between mufcular mo- 

 tions and Jenfual motions, or ideas, i. c fihey are both 

 originally excited by irritations. 2. And ajjociated to- 

 gether in the fame manner. 3. Both aft in nearly the 

 fame times. 4. Are alike ftrengthened or fatigued by 

 exercife. 5. Are alike painful from inflammation. 6. 

 Are alike benumbed by comprejjion. 7. Are alike liable 

 to paralyfis. 8. To convulfion. 9. To the influence 

 of old age. VI. Objections anfwered. i . Why we 

 cannot invent new ideas, i. If ideas refemble exter- 

 nal objefls. 3. Of the imagined fenfation in an ampu- 

 tated limb. 4. /ibftratl ideas. VII. What are ideas, 

 if they are not animal motions ? 



BEFORE the great variety of animal motions 

 can be duly arranged into natural clafles and orders, 

 it is neceflary to fmooth the way to this yet uncon- 

 quered field of fcience, by removing fome obftacles 



C which 



