OF SIGHT. (55 



transverse diameter, d e, which runs from the temporal to the 

 nasal side of the eye-hall, lies the axis of the organ in refer- 

 ence to the action of the superior and inferior straight muscles. 

 The perpendicular diameter passes from above downwards 

 through the point of revolution a, cutting the transverse 

 diameter at a right angle, and is at the same time the axis of 

 revolution of the internal and external straight muscles of the 

 eye. A line drawn from the outer margin of the cornea, f t to 

 the inside of the entrance place of the optic nerve, #, represents 

 the horizontal diagonal axis of the eye-ball, and is at the 

 same time the axis of revolution in reference to the two oblique 

 muscles. The superior oblique turns the pupil downwards 

 and outwards ; the inferior oblique turns it upwards and 

 outwards. The action of the whole muscles of the eye is pro- 

 ductive of no change in the position of the eye-ball, but only of 

 a revolution upon its axis. The faculty, however, which 

 enables us to judge of distances, and to adjust the eye so as to 

 obtain distinct vision at different distances, although it is pro- 

 bably only gradually acquired, is generally exerted uncon- 

 sciously. The power of thus accommodating the eye is pos- 

 sessed in very different degrees by different individuals ; it is 

 particularly remarkable in some of the higher animals ; and in 

 some men is either totally wanting or is reduced to a minimum. 

 Short-sightedness depends almost invariably on a loss of the 

 power of accommodation in the eye, as a consequence generally 

 of early and undue exercise of the organ upon objects close at 

 hand. This defect is therefore almost entirely confined to per- 

 sons in a certain rank of life, or having certain pursuits : the 

 majority of scholars and men of letters are short-sighted. In 

 the same way also far-sightedness is frequently an effect of the 

 want of the power of accommodation in the eye : sailors, who 

 are always looking at the horizon, are all but invariably far- 

 sighted. Both short-sightedness and far-sightedness are but 

 the limits to innumerable and individual departures from that 

 which may be held the standard in the structure of the eye. 



[ 134. There are many experimental ways of proving the 

 different positions which the images of near and distant objects 

 occupy upon the retina. One of the best known is that of 

 Scheiner,* which has been variously modified by different 



* Father Sclieiner made this experiment more than two hundred years 



7 



ago: RWJ ur^ta, &c 1G2G 29. 



