COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 7 



HISTORICAL. 



Marietta's discovery of the blind-spot seems to have played an 

 important part in instigating the examination of the peripheral retina. 

 It is, however, to Thomas Young that we owe the first published account 

 of an attempt to explore the retina with a view to determining the form 

 and extent of its sensitive surface. Troxler attacked the problem inde- 

 pendently a few years later and discovered that the periphery possesses 

 a characteristic which is absent from the central regions. After the 

 lapse of twenty years Purkinje confirmed and extended the work of 

 Troxler in an exceedingly important series of experiments ; but it was 

 not until the days of Szokalsky and Aubert that the problem of peri- 

 pheral as compared with central vision assumed definite shape, and that 

 the characteristic features of each were investigated in detail. 



In a paper read before the Royal Society of England, in 1800, 

 Thomas Young* communicated the following statement : 



The visual axis -being fixed in any direction, I can at the same time see a 

 luminous object placed laterally at a considerable distance from it; but in various 

 directions the angle is very different. Upwards it extends to 50 degrees, inwards 

 to 60, downwards to 70, and outwards to 90 degrees. These internal limits of 

 the field of view nearly correspond with the external limit formed by the different 

 parts of the face, when the eye is directed forwards and slightly downwards, 

 which is its most natural position. * * * The whole extent of perfect vision 

 is (little more than 10 degrees ; or more strictly speaking, the imperfection begins 

 within a degree or two of the visual axis, and at the distance of 5 or 6 degrees 

 becomes nearly stationary, until, at a still greater distance, vision is wholly 

 extinguished. The imperfection ds partly owing to the unavoidable aberration of 

 oblique rays, but principally to the insensibility of the retina. * * * The 

 motion of the eye has a range of about 55 degrees in every direction ; so that the 

 field of perfect vision, in succession, is by this motion extended to no degrees. 



Troxler-f- found that images which fall upon eccentric parts of the 

 retina fade out rapidly and soon disappear, while those upon the central 

 area of the retina persist. This peculiarity of indirect vision he dis- 

 covered accidentally while engaged upon an attempt to prove that the 

 blind-spot is not wholly insensitive to light, as its discoverer had stated. 

 Troxler had pasted a series of white paper figures upon a light-blue 

 background, and was surprised to find that when each was fixated 

 monocularly in turn, all of the others gradually paled out and finally 



*Thomas Young. On the Mechanism of the Eye, Philosophical Transactions, 

 XCII, 1801, pp. 23ff; Miscellaneous Works of Thomas Young, London, 1855, 

 pp. iaff. 



fD. Troxler. Ueber das Verschwinden gegebener Gegenstande innerhalb 

 unseres Gesichtkreises. Himly und Schmidt's Ophthalmologische Bibliothek, Jena, 

 1804, II, 2, S. 1-53- 



