466 SPECIAL PHYSIOLOGY. 



at the same rate, the image appears stationary; but if the number 

 of teeth, or the rate of velocity, differs in the two wheels, then a 

 revolving image is seen (Faraday). After looking for any length of 

 time, at objects in motion, the appearance of movement may be com- 

 municated even to stationary objects ; thus, after having been on the 

 sea, all surrounding objects appear, for a time, as if they were in a 

 state of constant upward and downward movement. 



When the retina has been previously in a state of repose, especially 

 in cases of prolonged residence in the dark, the influence of any given 

 quantity of light, as well as the rapidity of its action, and the dura- 

 tion of its impressions on the eye, are much greater than when it has 

 previously been in a state of activit}'. If the intensity of the light be 

 very great, the painful effect called dazzling, is produced, and the 

 nervous power of the retina may be permanently destroyed, as, for 

 example, when a flash of lightning suddenly annihilates vision. On 

 the other hand, blindness may result from the continuance of the op- 

 posite condition ; for if the eye be deprived of sufficient light, for a 

 very lengthened period, blindness will ensue from want of exercise of 

 the retina. If the eye, previously in a state of rest, be suddenly ex- 

 posed to a bright light, the rays impinging upon the retina through 

 the dilated pupil, are painful to the eye, and vision is confused, till 

 the pupil, having had time to contract, a large number of the rays are 

 excluded, and the retina itself, moreover, becomes accustomed to the 

 excess of light. When a sudden transition takes place from light to 

 darkness, opposite changes of course ensue. 



The relative intensity of light is measurable by means of instru- 

 ments called photometers, the action of which is, however, entirely 

 dependent on the discriminating power of the eye itself, through the 

 comparison of shadows of different strengths. We have no means of 

 estimating the absolute quantity or intensity of light. It is possible 

 to read with both eyes, during twilight, when the employment of one 

 eye would be useless ; and, moreover, a brilliant light dazzles, or 

 blinds, the two eyes, more rapidly than it does one. At the same 

 time, no difference of brightness is ordinarily observable, whether we 

 look with one eye or with both ; for on closing one eye, the pupil of 

 that eye dilates, and, consentaneously, that of the other, so that more 

 dight is admitted to the open eye. 



There are many interesting points connected with the so-called 

 secondary, or negative, ocular spectra^ which are distinguished from 

 the primary or positive spectra, by not resembling the original impres- 

 sion. If, after looking at dark, white, or luminous objects, the eyes 

 be covered, so as altogether to exclude the entry of light, images of 

 these objects, or impressions related to such images, the results of 

 after-sensations, remain upon the retina, and these are named negative 

 spectra. In the case just mentioned, of closing the eye, the spectra 

 are dark, white, or luminous, like the original causes of the visual im- 

 pressions. But if the eyes, after looking steadily at a white object or 

 spot, on a dark ground, instead of being closed, be turned towards a 

 white surface, the spectrum perceived is black. If the condition of 

 things be reversed, the spot being black, and the ground white, the 



