When the eve receives a succession of flashes of equal duration from a light of con stant intensity, which succeed each other so rapidly as to produce a uniform impression, the intensity of this aggregate impression will also be constant, provided the number of flashes in a given time varies inversely with the duration of each. The brightness of the impression produced by flashes of light of a given intensity, which succeed each other so rapidly as to produce a uniform impression on the eye, is proportional to the number of flashes in a given time.
When light of a given intensity acts on the eye for a short space of time, the brightness of the luminous impression on the retina is exactly proportional to the time during which the light continues to act. This law has been proved to be true for impressions lasting from to ,4 of a second.
The intensity of the impression produced by light, which acts on the eye for -a-, of a second, is almost exactly of the apparent brightness of the light when seen by uninter rupted vision ; and the time required for light to produce its full effect on the eye seems to be about of a second. Lights of different intensities produce their complete impressions on the eye in equal times, so that the light of the sun requires the same time as common artificial light to produce its impression on the eye. The brightness of an impression on the eye increases with a rapidity exactly pro portional to that of the light by which it is produced.
Rays of different refrangibility act on the eye with equal rapidity. The apparent bright ness of the spark produced by electricity of high tension is only about 0o of what its apparent brightness would become if its duration were prolonged to of a second ; and the brightness of electric light increases with the tension of the electricity.
The wonderful rapidity of the electric light is shown by the following experiments: —Viewed by the illumination of an electric spark, the spokes of a wheel in the most rapid rotation appear stationary, vibrating cords seem to be in a state of repose, and a succession of drops, which generally appear to the eye as a con nected stream, is seen to be but a succession of drops, because the impression of each image lasts for so short a time that the posi tion of the moving bodies is not altered. The light of electricity of high tension has a less duration than the millionth part of a second.
When on the subject of light, we have alluded to the reproduction of the impression of white light by the rapid revolution of a disk 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 white paper, and divides one into eight equal sectors, of which two 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, ha 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 plisses 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 eye 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 withs out my right eye if I did Lut intend my fancy a little while upon it : 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 belbre 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, I 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. 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 i is displayed to our view, and by an nstanta 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 mosphere and a sufficient amount of light are of importance, for every one must be aware how deceptive is the estimate of the size of an object when seen through a fog, or looming large in the gloom of the evening.