THE SENSE OF SIGHT 179 



and more remote objective points will appear faded and indistinct, for the 

 glass tablet is struck by the rays of the nearer object c before their union at the 

 point c' by the rays of the more distant object a, after their union at the point 

 a' , and consequently in both cases by rays diverging from one another. If we 

 move the tablet forward to a' the images of h and c fall within so-called circles of 

 dispersion, since the rays from both unite at a point behind the tablet. What 

 is here said of a single objective point holds good likewise for larger objects, 

 which are to be considered as composed of a great number of points lying near 

 one another. Wholly conformable to this is the process in the eye : the images 

 of several objects at different distances, simultaneously projected on the retina 

 by the cornea and crystalline lens, fall also behind one another, so that only one 

 of them can be in the plane of the retina, while the others fall either bef)re or 

 behind it, and thus present only the dim images of the dispersion circles. A 

 simple experiment shows this : if we close the left eye and hold before the right 

 at a certain distance a finger so that this finger shall be in a line with a remoter 

 object lying behind it, the window for instance of an opposite house, and then 

 fixedly regard now the finger and now the distant window, it will result that 

 whenever Ave thus regard the finger the window will appear indistinctly, with 

 dim outlines, and the finger indistinctly when we fix our attention on the win- 

 dow. Still more evident is the following experiment : Pierce with a needle two 

 small openings in a card near one another, hold the openings as closely as pos- 

 sible before the pupil of one eye, while the other eye is closed, and observe 

 through the openings alternately one and the other of two pins set up in front 

 of the eye at some distance from one another; the more distant pin will now 

 appear indistinct and double when the nearer one is atteutividy regarded, and 

 vice versa ; the pin to which the sight is directed appearing always wi^ll defined 

 and single. The double image of the indistinct pin arises simply from this, that 

 two separate pencils of rays proceed from it through the two apertures of the 

 card into the eye and strike the retina, which is occupied with the pin to which 

 attention is directed, either before their union at a single point or after it in two 

 separate places. The eye-mirror shows in the most direct manner the different 

 distinctness of the images formed on the retina by objects at different distances. 

 From the foregoing experiments we learn, first, that two objects at different 

 distances cannot be seen distinctly at the same time, and secondly, that we have 

 the power to see at will either the nearer or more remote object with distinctness ; 

 in other words, that we have the power to adjust our eye to different distances, 

 so that distinct images of objects unequally remote may ftiU just on the plane 

 of the retina. This power has received the name of the faculty of ocular ac- 

 commodation, and the alteration in question, which we execute for the adjust- 

 ment of the eye to different distances, is termed accommodation or adaption. 

 This faculty of accommodation is not unlimited; no eye is capable of being 

 adjusted to all possible distances from the most immediate proxim-ity to infinite 

 remoteness ; objects which are too near appear indistinct to all eyes in spite of 

 every effort of accommodation, as do objects removed beyond a certain distance 

 to most eyes; in other words, the images of objects which are too near fiill, in 

 the case of every eye, behind the plane of the retina, the images of objects too 

 remote are formed in front of the retina. The limits within which accommoda- 

 tion is possible are very different in different persons; and in the same person 

 may in the course of time be impaired or lost. Short-sighted persons are those 

 to whose eyes objects may be brought very close without becoming indistinct, 

 while remote objects are obscure; fiir-sighted persons, on the other hand, are 

 those who see clearly very distant objects, and to whom near ones become dim 

 at a relatively considerable distance. Between the normal eye and the object 

 about eight inches may intervene without indistinctness of vision f the near 

 sighted see clearly at four inches and even less. Constant occupation with ob- 

 jects which require close inspection produces shortness of sight; the contrary 



