CLASS I. 2. 5. 3, OF IRRITATION. 125 



observed, that if the common actions of the muscles affected the 

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

 to objects at any distance, must disturb our vision. At the same 

 time though it is possible that a violent action of all the muscles 

 together, so as to counterbalance each other, and keep the eye 

 motionless, as when we look painfully at a very distant object, 

 may in some degree affect the form of it; yet that the contrac- 

 tion of the ciliary process perpetually adapts the focus of the crys- 

 talline lens to the distance of the retina is not to be disputed. 



There have been instances of some, who could not distinguish 

 certain colours; and yet whose eyes, in other respects, were not 

 imperfect. Philos. Transact. Which seems to have been 

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

 some classes of fibres which compose the retina. Other perma- 

 nent defects depend on the diseased state of the external organ. 

 Class I.I. 3. 14. I. 2.2. 13. IV. 2. I. 11. 



3. Musccz volitantes. Dark spots appearing before the eyes, 

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

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

 the retina, which have been lately exposed to more luminous ob- 

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

 Hence dark spots are seen on the bed-clothes by patients, when 

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

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

 with their fingers to discover what they are; for these parts of 

 the retina of weak people are sooner exhausted by the stimulus 

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



Other kinds of ocular spectra, as the coloured ones, are also 

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

 and to produce various hallucinations of sight. For after the 

 contraction of a muscle, the fibres of it continue in the last situa- 

 tion, till some antagonist muscles are exerted to retract them; 

 whence, when any one is much exhausted by exercise, or by want 

 of sleep, or in fevers, it is easier to let the fibres of the retina re- 

 main in their last situation, after having been stimulated into 

 contraction, than to exert any antagonist fibres to replace them. 



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

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

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

 tivity of our sense of vision, besides that used in the motions of 

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

 the motions of our arms, which are supplied with nerves of 

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

 the light should be kept from patients in fevers with debility, to 

 prevent the unnecessary exhaustion of the sensorial power. 



