236 THE HUMAN BODY 



^5 of the whole, for all the range of lights we use in carrying 

 on our ordinary occupations. For strong lights the smallest per- 

 ceptible fraction is considerably greater; finally we reach a limit 

 where no increase in brightness is felt. For weak illumination the 

 sensation is more nearly proportioned to the total differences of 

 the objective light. Thus in a dark room an object reflecting all 

 the little light that reaches it appears almost twice as bright as 

 one reflecting only half; in a stronger light it would not so appear. 

 Bright objects in general obscurity thus appear unnaturally 

 bright when compared with things about them, and indeed often 

 look self-luminous. A cat's eyes, for example, are said to " shine 

 in the dark"; and painters to produce moonlight effects always 

 make the bright parts of a picture relatively brighter, when com- 

 pared with things about them, than would be the case if a sunny 

 scene were to be represented; by a relatively excessive use of 

 white pigment they produce the relatively great brightness of 

 those things which are seen at all in the general obscurity of a 

 moonlight landscape. 



Function of the Rods. Inasmuch as the rods are absent from 

 the fovese, they cannot be concerned with ordinary conscious vi- 

 sion since clear vision, as we know from experience, is confined to 

 these areas. It is easy to demonstrate by a simple experiment 

 that the parts of the retina containing rods are more susceptible 

 to feeble lights than is the fovea, which is devoid of them. The 

 constellation of the Pleiades consists of seven stars ; one of these is 

 so faint, however, as to be invisible to most eyes when the con- 

 stellation is looked at directly. If the gaze be turned to a point 

 in the sky a degree or two to one side of the constellation, so as 

 to throw its image off the fovea unto a rod-containing area, the 

 seventh star becomes immediately visible. 



This evidence indicates that the function of the rods is some 

 how related to the reception of light stimuli of feeble intensity. 

 The portions of the retina outside the fovea seem to function for 

 the most part more reflexly than consciously; stimuli striking 

 these portions of the retina bring about reflex movements of the 

 eyes and head so that the source of stimulation throws its light 

 upon the fovese, and we derive conscious perceptions as to its 

 nature. For such reflex activity a high degree of irritability is 

 desirable. 



