VISION. 



painted with the prismatic colours ; and Pla- 

 teau describes an experiment which leads to a 

 singular result. He takes two disks of exactly 

 the same size, made of thick while paper, and 

 divides one into eight equal sectors, of which 

 t\vo and two, corresponding and opposite ones, 

 are coloured red, white, blue, and black. The 

 second disk is coloured entirely black, two 

 sections lying opposite to one another, and 

 rather shorter and narrower than those of the 

 first disk, being cut out. Both disks are then 

 attached to rollers which are as much equal to 

 one another as possible. They are placed 

 vertically behind one another, so that- the 

 axes of rotation coincide, and the rollers are 

 set in motion by cords which pass over two 

 wheels, as nearly as possible equal to one 

 another. The posterior coloured disk, which 

 is rendered transparent by varnish, is well 

 lighted from behind by a lamp. On rotation of 

 the disks the whole field at first appears black, 

 but, by gradual transitions, it passes into red, 

 then white, and lastly into blue. 



The stimulus of vivid light produces an 

 effect upon the retina which is stronger and 

 lasts longer, in proportion to the intensity of 

 the primitive effect. The after-images of light 

 objects will be light, and those of dark objects 

 dark, if the eye be withdrawn from all subse- 

 quent action of light. If, for instance, we 

 look long through a window towards the clear 

 sky, and, turning .suddenly away, close the 

 eye, we shall see the light intervening spaces 

 bounded by the dark window frames ; but if, 

 on the contrary, the eye be turned towards a 

 white wall, the after-image of the frame will 

 appear light, and the intervening spaces dark. 

 The reason is, that if the eye, already dazzled, 

 1)2 turned towards the white surface, the parts 

 of the retina previously affected by the bright 

 light will be less sensitive to the white light of 

 the wall, than those parts on which the image 

 of the dark window frames has fallen, and 

 which has therefore not been unduly stimu- 

 lated. If the bright sun, or the intense light 

 resulting from the combustion of lime in 

 oxygen, be gazed upon, the spectrum continues 

 for a long period, and if the eye be closed, it 

 passes through a series of colours until it dis- 

 appears. The white is followed by yellow, 

 orange, red, green, violet, and finally black, 

 one after the other in regular succession. Sir 

 Isaac Newton experimented upon this subject, 

 and ran a great risk of blinding himself thereby, 

 as is described by him in a letter to John 

 Locke, dated June 30th, 1691. After detail- 

 ing the various steps of his experiment of 

 gazing on the sun, and observing the subse- 

 quent phantasm, he says, " At length by 

 repeating this, without looking any more on 

 the sun, I made such an impression on my 

 eye, that if I looked upon the clouds, or book, 

 or any bright object, I saw before it a round 

 bright spot like the sun ; and, which is still 

 stranger, though I looked upon the sun with 

 my right eje only, and not with my left, yet 

 my fancy began to make an impression upon 

 my left eye as well as upon my right. For if 

 I shut my right eye and looked upon a book 



or a cloud with my left eye, I could see the 

 spectrum of the same almost as plain as with- 

 out my right eye if I did Lut intend my fancy 

 a little while upon it t for at first if I shut 

 my right eye, and looked with my left, the 

 spectrum of the sun did not appear till I in- 

 tended my fancy upon it ; but by repeating 

 this it appeared every time more easily. And 

 now in a few hours' time I had brought my 

 eyes to such a pass, that I could look on no 

 bright object with either eye, but I saw the sun 

 before me, so that I durst neither write nor read: 

 but to recover the use of my eyes, shut myself 

 up in my chamber made dark for three days 

 together, and used all means to direct my 

 imagination from the sun ; for if I thought 

 upon him, I presently saw his picture, though 

 I was in the dark: but by keeping in the dark, 

 and employing my mind" upon other things, 

 began in three or four days to have some use 

 of my eyes again, and by forbearing to look 

 upon bright objects, recovered them pretty 

 well, though not so well but that, for some 

 months after, the spectrum of the sun began 

 to return as often as I began to meditate on 

 the phenomena, even though I lay in bed at 

 midnight with my curtains drawn/' 



Dimensions of Objects. With the represen- 

 tation of external things by means of the organ 

 of vision, the mind combines its knowledge of 

 their size and distance. We infer the real 

 magnitude of an object from its apparent mag- 

 nitude, or from the angle under which its rays 

 intersect each other in the eye ; our know- 

 ledge of its actual distance from us is merely a 

 deduction of our judgment arrived at through 

 the sensations excited in the eye according to 

 the different positions of objects, and such 

 sensations are regulated by the angle of vision. 

 For instance, we infer the increased distance 

 of an object of known size, as a man, from the 

 decrease of the visual angle. The angle of 

 parallax, or that angle which the axis of the 

 eyes when directed towards an object forms 

 with it at the point of contact, is also to be 

 taken into account, for it becomes greater in 

 proportion to the nearness of the object. We 

 are further aided by the position of the object 

 relatively to other known objects, but here, in 

 forming our conception of true dimensions, we 

 are largely assisted by the lessons of experi- 

 ence. The infant will grasp at the moon ; and 

 it is by degrees that it acquires such experi- 

 ence and judgment that the original percep- 

 tions of sight become signs of the tangible 

 qualities of external objects, and the distances 

 at which they are placed. In determining the 

 relative distances of objects one to another, 

 we are principally guided by the angle of vision ; 

 yet what an exercise of judgment is implied, 

 founded on a comparison of a variety of dif- 

 ferent circumstances, and involving a complex 

 mental operation, when a magnificent prospect 

 is displayed to our view, and by an instanta- 

 neous act of the mind we become aware of 

 the various distances at which all the com- 

 ponent parts are placed, the size of each indi- 

 vidual part, and the relation held by each to 

 the others. Here however, clearness of at- 

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