782 



THE EYE AND VISION 



[CH. LVI. 



some power from without; this power is supplied by the ciliary 

 muscle. Its action is to draw forwards the choroid, and by so 

 doing to slacken the tension of the suspensory ligament of the 



Fio. 584. Phakosc ;pe of Helmhultz. At />' I" ar<j two prisms, by which the light of a candle is con- 

 centrated on the eye of the person experimented with, which is looking through a hole in the third 

 iingle of the box opposite to the window C. A is the aperture fur the eye of the observer. The 

 observer notices three double images, represented by arrows, in tig. 583, reflected from the eye 

 under exiniination when the eye is tixed upon a distant object ; the position of the images having 

 been noticed, the eye is made to focus a near object, such as a reed pushed up at C ; the images 

 from the anterior surface of the lens will be observed to move as described m the text. 



lens which arises from it. The anterior surface of the lens is 

 kept flattened by the action of this ligament. The ciliary muscle 

 during accommodation, by diminishing its tension, diminishes to 

 a proportional degree the flattening of which it is the cause. On 

 diminution or cessation of the action of the ciliary muscle, the lens 

 returns to its former shape, by virtue of the elasticity of the suspen- 

 sory ligament (fig. 585). From this it will appear that the eye is 

 usually focussed for distant objects. In viewing near objects the 

 ciliary muscle contracts ; the ciliary muscle relaxes on withdrawal 

 of the attention from near to distant objects. 



It is possible to calculate the curvature of the lens or cornea in the body, by 

 measuring the size of the image of an object upon it The radius (r) of curvature 



of a convex reflecting surface is given by the formula r= ; a is the distance of 



C 



the object from the surface, b the diameter of the image, and c that of the object. 

 a and c are easily measured ; b is measured by Helmholtz's ophthalmometer, the 

 principle of which is as follows : If a line is looked at through a plate of glass 

 placed obliquely between it and the eye, the line is shifted sideways to either right 



