752 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



are both too near to have their image focussed upon the retina. The pin-hole 

 becomes itself a source of light, and appears as a luminous circle bounded by 

 the shadow thrown by the edge of the iris. Within this circle of light is seen 

 the shadow of the pin-head, but the pin-head appears inverted, for the obvious 

 reason that the eye, being accustomed to interpret all retinal impressions as 

 corresponding to objects in the opposite portion of the field of vision, regards 

 the upright shadow of the pin-head as the representation of an inverted object. 

 The course of the rays in this experiment is shown in Figure 217, in which 

 A B represents the card with a pin-hole in it, P the pin, and P' its upright 

 shadow thrown on the retina. 



Accommodation. From what has been said of conjugate foci and their 

 relation to each other it is evident that any change in the distance of the object 

 from the refracting media will involve a corresponding change in the position 

 of the image, or, in other words, only objects at a given distance can be 

 focussed upon a plane which has a fixed position with regard to the refracting 

 surface or surfaces. Hence all optical instruments in which the principle of 

 conjugate foci finds its application have adjustments for distance. In the 

 telescope and opera-glass the adjustment is effected by changes in the distance 

 between the lenses, and in the photographic camera by a change in the posi- 

 tion of the ground-glass plate representing the focal plane. In the microscope 

 the adjustment is effected by changing the distance of the object to suit the 

 lenses, the higher powers having a shorter " working distance." 



We must now consider in what way the eye adapts itself to see objects dis- 

 tinctly at different distances. That this power of adaptation, or " accommo- 

 dation," really exists we can easily convince ourselves by looking at different 

 objects through a network of fine wire held near the eyes. When with normal 

 vision the eyes are directed to the distant objects the network nearly disappears, 

 and if we attempt to see the network distinctly the outlines of the distant 

 objects become obscure. In other words, it is impossible to see both the 

 network and the distant objects distinctly at the same time. It is also evident 

 that in accommodation for distant objects the eyes are at rest, for when they 

 are suddenly opened after having been closed for a short time they are found 

 to be accommodated for distant objects, and we are conscious of a distinct 

 effort in directing them to any near object. 1 



From the optical principles above described it is clear that the accommo- 

 dation of the eye for near objects may be conceived of as taking place in three 

 different ways : 1st, By an increase of the distance between the refracting sur- 

 faces of the eye and the retina ; 2d, By an increase of the index of refraction 

 of one or more of the media ; 3d, By a diminution of the radius of curvature 

 of one or more of the surfaces. The first of these methods was formerly sup- 

 posed to be the one actually in use, a lengthening of the eyeball under a pres- 



1 It has been shown by Beer (Archivfilr die gesammte Physiologic, Iviii. 523) that in fishes 

 the eyes when at rest are accommodated for near objects, and that accommodation for distant 

 objects is effected by the contraction of a muscle for which the name "retractor lentis" is pro- 

 posed. 



