THE EYE. 



causes the eyelids to close ; and it is not until after the lapse of 

 a certain time that they can be opened without inconvenience. 



The cause of this is easily explained. While the observer re- 

 mains in the darkened or less illuminated room, the pupil is 

 dilated so as to admit into the eye as great a quantity of light as 

 the structure of the organ allows of. When he passes suddenly 

 into the strongly illuminated room, the flood of light arriving 

 through the widely dilated pupil acts with such violence on 

 the retina as to produce pain, which necessarily calls for the 

 relief and protection of the organ. The iris, then, by an 

 action peculiar to it, contracts the dimensions of the pupil so 

 as to admit proportionally less light, and the eye is opened with 

 impunity. 



Effects the reverse of these are observed when a person passes 

 from a strongly illuminated room into one comparatively dark, or 

 into the open air at night. For a certain time he sees nothing, 

 because the contraction of the pupil, which, was adapted to the 

 strong light to which it had previously been exposed, admits so 

 little light to the retina that no sensation is produced. The pupil, 

 however, after a while dilates, and, admitting more light, objects 

 are perceived which were before invisible. 



43. It is sometimes inferred, though erroneously, that the 

 apparent splendour of the image of a visible object decreases as 

 the square of the distance increases. This would be the case in 

 the strictest sense, if, while the object were withdrawn from the 

 eye to an increased distance, its image on the retina continued to 

 have the same magnitude ; for, in this ease, the absolute brightness 

 of each point composing such image would diminish as the square 

 of the distance increases, and the area of the retina over which 

 such points are diffused would remain the same ; but it must be 

 considered, that as the object retires from the eye the superficial 

 magnitude of the image on the retina is diminished in the same 

 proportion as the square of the distance of the object from the 

 eye is increased. It therefore follows that while the points com- 

 posing the image on the retina are diminished in the intensity of 

 their illumination, they are collected into a smaller space, so thf/t 

 what each point of the image on the retina loses in splendour, the 

 entire image gains by concentration. 



44. If the sun were brought as close to the earth as the moon, 

 its apparent diameter would be 400 times greater, and the area of 

 its apparent disk 160000 times greater than at present, but the 

 apparent brightness of its surface would not be in any degree 

 increased. In the same manner, if the sun were removed to ten 

 times its present distance, it would appear under a visual angle 

 ten times less than at present, as in fact it would to an observer 



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