640 PHYSIOLOGY 



going into a flower-garden on a summer morning, when dawn 

 is just beginning, although all objects in the garden can be clearly dis- 

 tinguished, there is a striking difference in its colour-tone as compared 

 with that which it presents in daylight. The scarlet geraniums have 

 disappeared. On close examination this disappearance is found to 

 be due to the fact that the flowers are dark, i.e. the light from them 

 does not stimulate the retina at all. The other coloured flowers can 

 be distinguished, but only in shades of grey. With a very little increase 

 in illumination the blue flowers come into evidence and the prevailing 

 tone of the garden is cold, made up as it is of greens, blues, and greys. 

 With increasing illumination the reds finally make their appearance. 

 After long exposure to the darkness of night the eyes have become 

 dark-adapted. The same behaviour of the dark-adapted eye may 

 be demonstrated in the laboratory. If a person who has been in a 

 dark room for half an hour observes a spectrum of low intensity the 

 whole spectrum appears colourless, its red end being cut off. The 

 distribution of luminosity over the spectrum is also altered. Whereas 

 the spectrum to the normal light-adapted eye appears brightest in the 

 yellow between the lines D and E, the spectrum of low intensity to the 

 dark-adapted eye has its point of greatest luminosity in the green. 



This striking difference between the light-adapted and the dark- 

 adapted eye does not apply to small objects the image of which, 

 when the vision is directed towards them, will fall entirely on the 

 fovea centralis of the retina. In the dark-adapted eye the 

 sensitiveness of the central spot of the retina is not nearly so 

 great as that of the more peripheral portion. On a dark night we 

 are often able to distinguish a star which, however, disappears as 

 soon as we turn our eyes so as to bring its image on the central 

 spot of the two retinae. Moreover the qualitative change in relation 

 to the colours observed in the dark- adapted eye does not apply 

 to the fovea centralis. In a dark room a small spot of light, whatever 

 its colour, when the visual axes are directed on it, is seen in its true 

 colour as soon as its intensity is sufficient for it to be seen at all. Before 

 this takes place it may be seen by the peripheral parts of the retina, but 

 as a colourless spot of light which disappears as soon as the gaze is 

 directed on it. This marked difference between the behaviour of the 

 foyea centralis and the more peripheral parts of the retina in the dark- 

 adapted eye has been attributed to the difference we have already 

 studied in the anatomical structure of these parts. The only per- 

 cipient elements found in the fovea centralis are the cones. In the 

 adjacent portion of the retina we find also the rods which increase in 

 relative number as we pass from the central spot towards the periphery. 

 Schultz long ago pointed out that in the retinae of many night animals, 

 such as the owl, the mouse, the, ea,t, the rods are, the predominating 



