VISUAL SENSATIONS. 1055 



shortens at this end with increasing age, apparently owing to increased 

 pigmentation. 



According to Helmholtz, 1 the ultra-violet spectrum as far as the line 

 R, is visible under normal conditions, being indigo blue with slight and 

 bluish grey with greater intensity. Its visibility is said to be increased 

 by removal of the lens, 2 and it is said that this part of the spectrum 

 then appears nearly or quite colourless. In cases in which the lens has 

 been removed, it is also said that Eb'ntgen rays give rise to a visual 

 sensation, but the evidence for this is unsatisfactory. The invisibility 

 of the infra-red rays is probably due to insensitiveness of the retina, 

 while the ultra-violet rays fail to be seen, partly, at any rate, owing to 

 absorption by the ocular media. 



The exact limits of the visible spectrum show a certain amount of 

 variation in individuals with normal vision, and greater variations in 

 abnormal vision. 



Visual acuity. This is usually denned by the smallest visual 

 angle which the line between two points must subtend, in order that 

 the points may be distinguished as two. It has also been defined by 

 the smallest angle at which the eye can recognise the form of an object. 

 In practice it is most commonly tested by the power of distinguishing 

 letters, the visual acuity being expressed by the formula V J, where D 

 is the distance at which the letters employed subtend an angle of 

 five minutes, and d is the greatest distance at which these letters can be 

 recognised, thus V=4 means that letters which subtend an angle of 

 five minutes at 5 metres can be read at 5 metres. Such a method, though 

 generally in use, is not satisfactory physiologically, for the perception of 

 the form of relatively complicated objects like letters involves other than 

 retinal factors, and Guillery 3 has proposed as the test of visual acuity 

 the smallest extent of a black point on a white ground which can be 

 perceived at a given distance, and finds that this method gives satis- 

 factory results. Such a point has been called the " physiological 

 point " (Aubert). The subject of visual acuity is one of great practical 

 importance, for it has become customary to use it much more largely as 

 a test of the condition of the retina than either the light or colour sense. 



The relation between visual acuity and intensity of illumination 

 has been much investigated. The acuity increases with increased 

 brightness, and this fact has been made the basis of a method of colour 

 photometry (p. 1077). According to Posch, 4 visual acuity increases 

 rather more quickly than the logarithm of the intensity of illumination, 

 and Konig 5 has recently found a similar logarithmic relation. Konig 

 also finds that at very low luminosities the relation between acuity and 

 intensity differs from that at ordinary luminosities. 



Extension and sensibility. There is a definite relation between 

 the extent of the retina stimulated and the brightness of the result- 

 ing sensation. Forster 6 was the first to point out that brightness 

 and visual angle supplement each other ; that the smaller the one the 

 greater must the other be in order that perception shall take place. 

 According to Charpentier, 7 this relation only holds for areas below a 



1 " Handbuch d. physiol. Optik," 2te Aufl., S. 279. 



2 Compt. rend. Acad. d. sc., Paris, 1883, tome xcvi. p. 509. 



3 Arch.f. Augenh., Wiesbaden, 1891, Bd. xxiii. S. 323. 4 Ibid., 1876, Bd. v. S. 14. 



5 Sitzungsb. d. k. Akad. d. Wissensch., Berlin, 1897, S. 559. 



6 "Ueber Hemeralopie u. die Anwendung eines Photometers," Breslau, 1857. 



7 " La lumiere et les couleurs," Paris, 1888, p. 138. 



