126 Remarks on Railway and Marine Signals. 



in the negative ; but in a relatively large proportion the 

 incapacity would be detected only by a detailed and sys- 

 tematic examination. 



It is, of course, well known that one condition essential to 

 distinct vision is that an image of the object looked at 

 should be formed on the retina. 



As regards refraction, it is now generally accepted that the 

 dioptric media in the normal eye accurately focus parallel 

 rays on the percipient layer of the retina ; consequently 

 neither divergent nor convergent rays can be brought to a 

 focus on that percipient layer without some alteration. 



For divergent rays this alteration is effected by an increased 

 convexity of the crystalline lens, produced through the 

 agency of the ciliary muscle. 



Hooke* investigated the angular distance required to ob- 

 serve two fixed stars separately, and he found that among a 

 hundred persons scarcely one was in a position to distinguish 

 the two stars when the apparent distance is less than 60". The 

 correctness of this observation of Hooke has been confirmed 

 in different ways by modern investigators. Professor Snellen, 

 of Utrecht, some years ago devised a series of black letters 

 on a white ground, which are easily read in good light by 

 the normal eye at such a distance that the whole letter is 

 seen under an angle of 5', but the openings in the letters 

 under an angle of T. These test types have come into 

 general use by those concerned in the management of optical 

 defects of the eye. It is necessary that the results of an 

 examination by the types should be further supplemented by 

 determination of the refraction, because it is possible for a 

 myopic, "J* or short-sighted person, sometimes to read No. 20 

 Sneller at 20 feet by partly closing the eyelids, so as to 

 diminish the circles of dispersion on the retina (and, perhaps, 

 by, at the same time, slightly flattening the eye), and, on the 

 other hand, a hypermetropic!" person, with good accom- 

 modation, may also read the same letters — viz., No. 20 at 

 20 feet. 



There is yet another anomaly of refraction — viz., astig- 

 matism, in which, owing to the curvatures of the dioptric 

 system being unequal in the different meridians, no true 



* Posthumous Works (1705), quoted by Professor Donders. 

 fin whom parallel rays are brought to a focus in front of the retina. 

 I In whom parallel rays, if continued, would come to a focus only behind 

 the retina. 



