556 



PHYSIOLOGY 



the red and violet ends. Since the band is colourless, any one part may 

 be matched by any other part by suitably adjusting the intensities. But 

 compared with the appearance under ordinary intensity, the red region 

 of the spectrum has become greatly reduced in visibility, while the blue has 

 become relatively brighter. The part of the spectrum with maximum 

 luminosity is found to be the yellow when the intensity is high, but to be 

 the green when it is low. It is therefore this shifting of the position in the 

 spectrum of the maximum which has caused red to darken and blue to 

 become lighter. The relative forms and positions of the luminosity (apparent 

 brightness) curves for spectra of various intensity are shown in Fig. 286. 



3-8 

 3C 

 34 

 3-2 

 3-0 

 2-8 

 2-6 

 2-4 

 2-2 

 2-0 

 1-8 

 1-6 



1-2 

 1-0 

 08 

 0-6 

 0-4 

 0-2 



870 650 625 605 590 575 555 

 B C D 



535 520 

 E 



505 490 



470 



450 



430 

 C 



Kn:. L'Hl). Luminosity curves for spectra of different intensity. A highest, 

 H = lowest. Abscissae = wavelengths, ordinates luminosities (KoNio). 

 The maximum of curve for light of hii^h intensity is seen (<> he at liloo A. I'. 

 that at low intensity 5150 A.IT. 



If the spectrum of low intensity be still further decreased, a point will 

 be reached at which the different parts become invisible to the eye ; this 

 will occur first with the ends, and last with the middle (at about 5271 A.U.). 

 It is found however that the intensity values at which visibility ceases 

 decrease the longer the eye is kept in the dark, that is to say the retina 

 gradually becomes 'dark adapted.' The curves obtained for different 

 degrees of dark adaptation are also shown in Fig. 286. 



We must now consider the effects on the achromatic threshold 

 of size of light source, duration of stimulus and part of retina illuminated. 

 With regard to size, experiment shows that as the size of the 

 source decreases so the intensity at which extinction occurs increases, in 

 fact that the area of the source multiplied by its intensity is constant. 

 With regard to the region of the retina that is stimulated, it is found that the 



