120 COLOUR VISION 



round, but differences in size are still appreciable. If the objects are 

 of equal size the brighter appears the larger, i.e., the apparent size 

 varies with the light intensity. Schoute, like v. Helmholtz, attributes 

 this to psychological causes, i.e., to an error of judgment. 



Loeser 1 has entirely confirmed Ricco's law for foveal vision. The 

 law is stated in one form thus : The product of the minimum visual 

 angle and the square root of the light intensity is constant. Loeser's 

 results are shown in the following table : 



Distance of Diameter of Visual angle, Sq. rt. of light Product, 



object, E object, D D/E intensity, J DJ/E 

 m. mm. 



8 20-0 2-5 0-87 2'18 



14-0 1-75 1-27 2-22 



8-5 1-06 ,2-4 2'5 



5-0 0-63 3-45 2'26 



The photochromatic interval (v. p. 60) can be demonstrated by 

 altering the area of the retina stimulated. Thus Bonders 2 found that 

 in full daylight the hue of intensely coloured papers on a dark back- 

 ground could be distinguished when they subtended a visual angle of 

 0'7 minute (1 sq. mm. at 5 metres distance). 



The dependence of the discrimination of hue on visual angle is 

 readily demonstrated with relatively unsaturated colours. Coloured 

 objects on a white background appear dark or grey under the smallest 

 angles at which they are visible. It is possible to select a grey 

 background of such a luminosity that the colourless interval is 

 abolished, i.e., as the visual angle is increased the colour of the object 

 is recognised as soon as the object becomes visible. 



Charpentier 3 determined the absolute and the chromatic thresholds 

 from the area stimulated for the fovea of the dark-adapted eye. Since 

 there is little adaptation at the fovea, this factor is of relatively slight 

 importance. He obtained the following results, the measurements 

 being the diameters of the diaphragm of the photometer. 



Colour (sunlight spectrum) Absolute Chromatic Ratio 



Threshold Threshold 



Extreme red . . . . - 5 mm. 1 mm. 4. 



Orange 0'9 2'1 5'5 



Yellow 1 3-1 9-6 



Green 0"3 4'2 196 



Blue.. .. 0-3 7-5 625 



1 Hirschberg's Festschrift, 1905 ; Feilchenfeld and Loeser, Arch. f. Ophlk. LX. 97, 1905. 



2 Ann. cTocul. LXXIX. 1878. 3 La Lumiere ct Us Coideurs, pp. 213, 238. 



