ICT. III. 5. MOTIONS OF THE RETINA. 27 



the flick is the fame that the obferver ufes in chang- 

 ing his ideas : thus the ^x, ootMW a w of Homer, the 

 long (hadow of the flying javelin, is elegantly de- 

 figned to give us an idea of its velocity, and not of 

 its length. 



4. The fatigue that follows a continued attention 

 of the mind to one object is relieved by changing 

 the fubjeft of our thoughts ; as the continued 

 movement of one limb is relieved by moving an- 

 other in its (lead. Whereas a due exercife of the fa- 

 culties of the mind ftrengthens and improves thofe 

 faculties, whether of imagination or recolle&ion ; 

 as the exercife of our limbs in dancing or fencing 

 increafes the ftrength and agility of the mufcles thus 

 employed. 



5. If the mufcles of any limb are inflamed, they 

 do not move without pain ; fo when the retina is 

 inflamed, its motions alfo are painful. Hence light 

 is as intolerable in this kind of ophthalmia, as pref- 

 fure is to the finger in the paronychia. In this dif- 

 eafe the patients frequently dream of having their 

 eyes painfully dazzled; hence the idea of ftrong 

 light is painful as well as the reality, The firft of 

 thefe facts evinces that our perceptions are motions 

 of the organs of fenfe; and the latter, that our 

 imaginations are alfo motions of the fame organs. 



6. The organs of fenfe, like the moving mufcles, 

 are liable to become benumbed, or lei's fenfible, 

 from comprefllon. Thus, if any perfon on a light 

 day looks on a white wall, he may perceive the ra- 

 mifications of the optic artery, at every pulfation 

 of it, reprefented by darker branches on the white 

 wall ; which is evidently owing to its comprefling 

 the retinue during the diaftole of the artery. Sa- 

 vage Noiblog. 



7. The organs of fenfe and the moving mufcles 

 are alike liable to be affe&ed with palfy, as in the 



D gutta 



