Chap, xxix.] THE VISUAL ANGLE. 383 



the eye. Thus one body larger than another, but at 

 a greater distance from the eye, will be seen under 

 the same angle x. The smallest visual angle per- 

 mitting distinct vision is 60 sec., and it corresponds 

 to a retinal image about 0'004 mm. in size, a size 

 just sufficient to cover one of the cones of the retina. 

 Two points seen under an angle of 60 sec. would 

 appear as one. 



The smaller the visual angle under which distinct 

 vision is possible, the more acute is the vision, so 

 that acuteiiess of vision is inversely as the size of the 

 visual angle. Test types now in use for estimating 

 acuteness of vision are constructed on this principle. 

 Thus, Snellen's types are all arranged to be seen under 

 an angle of o minutes. Let D be the distance at which 

 the types ought to be seen under the angle of 5 

 minutes, and d the shortest distance at which the 

 person whose sight is being tested sees the object, 

 then the acuteness of vision is given by the formula 



When d = D, acuteness of vision is normal. 



Accommodation of the eye for distance. The 

 refractive media of the eye are such that parallel rays 

 are brought to a focus on the retina ; the posterior 

 principal focus, that is to say, is on the retina. Such 

 an eye is called emmetropic. It is evident that if 

 divergent rays fall upon the eye, that is, rays from a 

 finite distance, they will not be brought to a focus 

 on the retina, but behind the retina, if the eye 

 remains in the same condition so far as its refraction 

 is concerned. The result of this would be circles of 

 diffusion, and a blurred and indistinct image. The 

 experiment of Schemer illustrates the diffusion 

 images. A card is taken, in which two small holes 



