1056 VISION. 



certain limit, corresponding approximately to the dimensions of the fovea. 

 Not only is the threshold of stimulation thus influenced by the size of 

 the retinal area, but also the threshold of discrimination for light and 

 colour. Donders 1 proposed to use the visual angle as the readiest 

 means of measuring the colour sense, and Guillery has recently applied this 

 method in the investigation of the sensitiveness of different parts of the 

 retina (p. 1086). Within certain limits it seems as if a definite amount 

 of light stimulating the retina produces the same sensory effect, whether 

 it is spread over many or concentrated on a few retinal elements. 2 



It has been found that when coloured light stimulates a very small 

 area of the retina, it may appear colourless, i.e. there is a a photo- 

 chromatic interval for stimulation of small extent. This was investigated 

 especially by Aubert, 3 who found the appearance of colour depended 

 greatly on the nature of the background, — yellow appearing very early 

 on a dark, very late on a bright, background, while green appeared 

 equally soon on both. The disappearance of the colour of small areas 

 depends partly on the intensity of the colour, and has been made the 

 basis of a method of colour photometry. The question is, however, 

 complicated by the difference between vision with the fovea and with 

 the surrounding part of the retina. 



If a small field of coloured light is reduced in size till it appears 

 colourless, it has been found that its colour may be restored, if other 

 equally small areas are simultaneously illuminated with the same light. 4 

 It seems as if a subliminal excitation in one retinal area were capable of 

 being reinforced by similar excitations in other parts of the retina (see 

 p. 1066). 



Adaptation and Induction. 



Adaptation. — The two chief factors on which the excitability of a 

 given area of the retina depends, are the nature of the preceding 

 stimulation and the condition of the surrounding retina. 



The condition of excitability of the retina, depending upon the nature 

 of the stimulation, is called adaptation. The visual apparatus has the 

 power of adjusting itself to differences of illumination, just as it has to 

 differences of distance. The adjustment to differences of illumination 

 depends on two factors, — the size of the pupil, and the condition of 

 excitability of the retina. The relation between the condition of the 

 pupil and retinal adaptation is not yet quite clear . Schirmer 5 has shown 

 that the two are connected, the size of the pupil depending on the 

 illumination to which the eye has previously been exposed, as well as 

 on the existing illumination (see p. 1043). 



Eetinal adaptation was first studied by Aubert, 6 who investigated the 

 extent to which the sensitiveness to minimal stimulation was affected 

 by complete exclusion of light from the eye. He found that the increase 

 was rapid at first, the sensitiveness being multiplied fifteen to twenty 

 times in the first two minutes. The increase took place more slowly 

 later, and the sensitiveness had almost reached its maximum in about 



1 Arch.f. Ophth., 1877, Bd. xxiii. Abth. 4, S. 282. 



2 Abney has recently (Proc. Roy. Soc. London, 1897, vol. lxi. p. 330) found a definite 

 relation between the threshold and the diameter of the retinal area stimulated, the smallest 

 diameter, and not the area, being the determining factor. 



3 "Physiol, d. Netzhaut," Breslau, 1865, S. 108. 



4 See A. E. Pick, Arch.f. d. ges. Physiol., Bonn, 1888, Bd. xliii. S. 445. 



5 Arch. f. Ophth., 1894, *Brl. xl. Abth. 5, S. 8. 



6 "Physiol, d. Netzhaut," Breslau, 1865, S. 25. 





