552 OF SENSATION". 



tion of their real comparative sizes from their apparent sizes is easily 

 effected by a simple process of mind ; but this is not the case, when we 

 only guess at their distances ; and our estimate of the size of objects, 

 even moderately remote, is as much affected by states of the atmosphere 

 as that of their distance, the one being, in fact, proportional to the 

 other. Thus a slight mist, which gives the idea of increased distance, 

 will also augment the apparent size ; because in order that an object 

 two miles off, should produce a picture upon the retina of the same 

 extent with that made by an object one mile off, it must have double 

 the dimensions. It is evident that our perception of the size of objects 

 must be acquired by experience, in the same manner as that of their 

 distance has been shown to be. 



969. We have now to consider briefly some other phenomena of 

 Vision, in which the acts of Mind, that have been just alluded to, do 

 not participate. The contraction of the Pupil, under the stimulus of 

 light, seems to be effected by a sphincter muscle, which surrounds the 

 orifice, and which is put in action by a branch of the Third pair of 

 nerves. This is an action with which the will has nothing to do ; and 

 it takes place entirely without our consciousness. Although it is due 

 to the stimulus of light, yet there is reason to believe that the conscious- 

 ness of the presence of light is not requisite ; and that it is, therefore, 

 a purely reflex action. The Optic nerve seems to be the channel 

 through which the impression is conveyed to the nervous centres ; 

 whilst the Third pair is that through which the motor impulse is con- 

 veyed to the iris ; but there is some ground for the idea that the Fifth 

 pair may in some degree convey the requisite stimulus, when the optic 

 nerve has been divided. That the dilatation of the pupil is a muscular 

 action, appears probable from the fact that the radiating fibres of the 

 iris are of the same character with the circular; both sets constituting, 

 in Man, a peculiar variety of the non-striated form of muscular tissue. 

 Through what nervous channel, however, the stimulus to this action is 

 conveyed, has not yet been clearly made out. The contraction of the 

 pupil is evidently destined to exclude from the interior of the eye, such 

 an amount of light as would be injurious to it ; whilst its dilatation in 

 opposite circumstances admits the greatest possible number of rays. 

 There is a contraction of the pupils, however, which takes place without 

 any change in the amount of light. This occurs when the two eyes are 

 made to converge strongly upon any object brought very near them ; 

 and its purpose appears to be, to prevent rays from entering the eye 

 at such a wide angle, as would render it impossible for them to be all 

 brought to their proper foci, and would thus produce an indistinct 

 image. 



970. In the use of the Eye, like that of the Ear, there is a tendency 

 to blend into one continuous image a succession of luminous impressions 

 made at short intervals ; upon which fact depend a number of curious 

 optical illusions. The length of the greatest interval that can elapse 

 without an interruption of the presence of the image (in other words 

 the duration of the visual impression), may be measured by causing a 

 luminous object to whirl round, and by ascertaining the longest period 

 that may be allowed for each revolution, consistently with the complete- 



