16 MOTIONS OF Sect. III. 5. 4. 



mufcular motion of the eye-lid is performed quicker than the 

 perception of light can be changed for that of darknefs.— -So if 

 a fire-Hick be whirled round in the dark, a luminous circle ap- 

 pears to the obferver ; if it be whirled fomewhat flower, this 

 circle becomes interrupted in one part ; and then the time taken 

 %»p in fuch a revolution of the flick is the fame that the obferver 

 ufes in changing his ideas : thus the "hoXiy^y.^ ty%o$ of Homer, 

 the long iliadow of the flying javelin, is elegantly defigned 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 fubjetl: of our thoughts; 

 as the continued movement of one limb is relieved by moving 

 another in its ftead. "Whereas a due exercife of the faculties of 

 the mind flrengthens and improves thofe faculties, whether of 

 imagination or recollection ; as the exercife of our limbs in 

 dancing or fencing increafes the ftrength and agility of the muf- 

 cles thus employed. 



5. If the mufcles of any limb are inflamed, they do not move 

 •without pain 5 fo when the retina is inflamed, its motions alfo 

 are painful. Hence light is as intolerable in this kind of oph- 

 thalmia, as preiTure is to the finger in the paronychia. In this 

 difeafe the patients frequently dream of having their eyes pain- 

 fully dazzled ; hence the idea of ftrong light is painful as well 

 as the reality. The firft of thefe facls evinces that our percep- 

 tions 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 mufcle9, are liable to 

 become benumbed, or lefs fenfible, from cornpreflion. Thus, 

 if any perfon on a light day looks on a white wall, he may per- 

 ceive the ramifications 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 retina during the diaftole 

 of the artery. Sauvages Nofolog. 



7. The organs of fenfe and the moving mufcles are alike lia- 

 ble to be affected with palfy,.as in the gutta ferena, and in fome 

 cafes of deafnefs j and one fide of the face has fometimes loft 

 its power of feniation, but retained its power of motion ; other 

 parts of the body have loft their motions, but retained their fen- 

 fation, as in the common hemiplegia ; and in other inftances 

 both thefe powers have perilhed together. 



8. In fome convulfive difeafes a delirium or infanity fuper- 

 venes, and the convulfions ceafe ; and converfely the convul- 

 Jions (hall fupervene, and the delirium ceafe. Of this I have 

 been a witnefs many times in a day in the paroxyfms of violent 

 fpilepfies ; which evinces that one kind of delirium is a convul- 



fion 



