74 COLOUR VISION 



Zahn 1 has investigated the " minimal-time luminosities.'' In these 

 experiments the coloured light is exposed momentarily, when it appears 

 colourless, and is compared in brightness with a surrounding white 

 surface. The minimal-time luminosities completely confirm the results 

 obtained by minimal fields and periphery values, as is shown by the 

 following values : 



Wave-lengths 659 621 601 589 564 542 523 506 



Minimal-time 

 luminosities 225 79'0 105'0 100 76'6 59'4 38'0 15'2 



Minimal-field 



luminosities 24'4 70'2 104'8 100 74-4 58'3 37'5 20'2 



Periphery 

 values 2T6 73'9 99'6 100 79'9 54'7 36'7 15'2 



Hess 2 , Hering 3 , and Tschermak 4 have examined the relative lumin- 

 osity of different lights in different regions of the retina. Hess found 

 that red and green pigments on a grey background, when viewed by 

 dull daylight with moderate dark adaptation, became darker and brighter 

 respectively in peripheral as compared with central vision. Hering 

 confirmed this result with spectral lights. Tschermak used Hering's 

 ; ' double room " (v. p. 58) with spectral lights from Auer-gas or arc light 

 against a daylit background. He found a relative increase in luminosity 

 from 516 to 466 /A/Z, a relative decrease from 693 to 525 /JL/JL, and no 

 change from 525 to 516 /A/A, on indirect fixation. In the dark-adapted 

 eye there is similarly a change in relative luminosity, and it occurs in 

 the same sense as the Purkinje phenomenon. Hering 5 has shown that 

 for the dark-adapted eye, even on momentary dark adaptation, lights 

 of equal brightness by direct fixation appear very unlike in luminosity 

 on indirect fixation. He describes these alterations in luminosity as 

 Purkinje's phenomenon produced by change of position in the visual 

 field without change of light intensity. 



The limits of the colour fields of the partially dark-adapted eye for 

 spectral colours have been worked out very thoroughly by Abney 6 . 



1 Ztsch.f. Sinnesphysiol. XLVT. 287, 1911. 2 Arr.fi.f. Ophth. xxv. 4, 1, 18S9. 



3 Arch./, d. gcs. Physiol. XLVII. 417, 1890. 



4 Ili'uL LXXXH. 559, 1900. 5 Ibiil. LX. 519. 1895 

 6 Abney. p. 190; Phil. Trnns. Roy. Soc. cxr. 155, 1897. 



