THE ORGANS OF SENSE. 123 



at this range the distance between adjacent dots or the segments of the 

 various letters, as the case may be, subtends visual angles of known 

 size. The visual acuity is expressed as a fraction, of which the 

 numerator is the distance in metres at which the test is made, and 

 the denominator is the distance at which the smallest type read 

 should be distinguished by an emmetropic eye, as, for example, 6/6 

 (normal), 6/9, 6/12, and so on. 



ACCOMMODATION. 



An emmetropic eye at rest is in focus for parallel rays, that is, for 

 rays coming from a distance of 6 metres or more. The images of 

 objects within this distance are brought to a focus on the retina by 

 an effort of accommodation. The essential part of the act of accom- 

 modation is that the crystalline lens becomes more convex, so that 

 divergent rays are brought to a 

 focus on the retina. On looking 

 obliquely into an eye which is 

 being accommodated for a near 

 object, the alteration in shape of 

 the lens can be observed. The 

 iris can be seen to move forward 

 because of the increasing con- FJG> 3tii _ 1 Sanson , s imageSj (1) tith eye 

 vexity of the lens. The altera- at rest, (2) during accommodation, 



tion in shape is limited to the (Sterling* Principles of Physiology.) 



, ,, . a, images from anterior surface of cornea: b. 



anterior Surface, and this can from anterior surface of lens; c, from 



-, j j , posterior surface of lens. 



be demonstrated by means or 



Sanson's images, which are most conveniently observed with the help of 

 an instrument known as the phakoscope. This consists of a triangular 

 box with truncated angles. At one angle is an aperture for the eye 

 of the observer, at another an opening for the observed eye, at the 

 third two triangular openings for the admission of light. Opposite 

 the observed eye is another opening in which a wire is placed. The 

 person who is being observed first relaxes his accommodation by looking 

 through the aperture opposite to him at an imaginary distant object. 

 The source of light is seen to be reflected from his eye at three positions, 

 namely, the anterior surface of the cornea, and the anterior and posterior 

 surfaces of the lens (Sanson's images). The images reflected from the 

 anterior surfaces of the cornea and lens are erect ; that from the 

 posterior surface of the lens is small and inverted. If the observed eye 

 be accommodated for the wire opposite it, the middle image only is 

 altered, coming nearer the anterior image and becoming smaller in size 

 (fig. 36). This shows that the anterior surface of the lens moves forward 



b 



