CI.ASS I. 2. 5. 3. OF IRRITATION. 125 



oofen ' common afHons of the mufcles affected the 



focus of the eye, every motion of the eye-ball, when we attend 

 to objects at any diflance, mud diilurb our vifion. At the fame 

 time though it is pofiible, that a violent action of ail the mufcles 

 together, fo as to counter-balance each other, and keep the eye 

 motionlefs, as when we look painfully at a very diftant object, 

 may in fome degree afreet the form of it ; yet that the contrac- 

 tion of the ciliary procefs perpetually adapts the focus of the cryf- 

 talliiie lens to the diftance of the retina is not to be difputed. 



There have bcen.inftances of fome, who could not diftinguifii 

 certain colours ; and yet \\hcfe eyes, in other refpecls, were not 

 imperfect. Philof. Traufact. Which feems to have been 

 owing to the want of irritability , .or the inaptitude to action, of 

 fome clafles of fibres which compofe the retina. Other perma- 

 nent defects depend on the difeaied ftate of the external organ, 

 i. i. 3. 14. 1. 2. 2. 13. IV. 2. i. n. 



3. MnfLiE wiitantes. Dark fpots appearing before the eyes, 

 and changing their apparent place with the motions of the eyes, 

 are owing to a temporary defect of irritability of thole parts of 

 jthe retina, which have been lately expofed to more luminous ob- 

 jects than the other parts of it, as explained in Seel. XL. 2. 

 Hence dark fpots are feen cm the bed-clothes by patients, when 

 the optic nerve is become lefs irritable, as in fevers with great 

 debility j and the patients are perpetually trying to pick them off 

 with their fingers to difcover what they are ; for thefe parts of 

 the retina of weak people are fooner exuaufted by the ftimulus 

 of bright colours, and are longer in regaining their irritability. 



Other kinds of ocular fpectra, as the coloured ones, are alfo 

 more liable to remain in the eyes of people debilitated by fevers, 

 and to produce various hallucinations of fight. For after the 

 contraction of a mufcle, the fibres of it continue in the kft fitu- 

 ation, till fome antagonift mufcles are exerted to retract them ; 

 whence, when any one is much exhaufted by exercife, or by want 

 of deep, or in fevers, it is eafier to let the fibres of the retina re- 

 main in their lad fituation, after having been ftimulated into 

 contraction, than to exert any^antagoniit fibres to replace them. 



As the optic nerves at their entrance into the eyes are each of 

 th,em as thick as a crow-quill, it appears that a great quantity of 

 fenforial power is expended during the day in the perpetual ac- 

 tivity of our fenfe of viiion, befides that ufed in the motions of 

 the eye-balls and eye-lids ; as much I fuppofe as is expended in 

 the motions of our arms, which are fupplied with nerve ; of 

 about the fame diameters. From hence we may conclude, that 

 the" light fhould be kept ,rom patients in fevers with debility, to 

 prevent the unneceifary exh ; of the fenfcvial power. 



And 



