SENSE OF SIGHT. 627 



seconds. In the observations made upon fixed stars, it is found that 

 two stars can never be separately distinguished by the eye unless their 

 angular distance from each other is equal to 30 seconds ; and very 

 seldom, unless it be as great as 60 seconds. These measurements 

 correspond with each other only in an approximative manner ; perhaps 

 because there has never been an opportunity of examining the retinal 

 elements in an eye, of which the acuteness of vision has been tested 

 beforehand. But they are sufficient to indicate a probable connection 

 between the minute structure of the retina and the possible limit of its 

 sensibility to separate impressions. 



Physiological Conditions of the Sense of Sight. The apparatus of 

 vision, as above described, consists of various parts, each of which has 

 its appropriate share in producing the final result of visual perceptions. 

 The eye, so far as regards its physical structure, is an optical instrument, 

 composed of transparent and refracting media, a perforated diaphragm, 

 and a dark chamber lined with a blackened membrane, all of which act 

 upon the luminous rays according to the same laws as the corresponding 

 parts in a telescope or a camera ; and the accuracy of their adjustment 

 is one of the first requisites for the exercise of sight. The organ, 

 furthermore, is movable as a whole; and certain of its internal parts 

 are also under the control of muscular tissues, whose alternate con- 

 traction and dilatation contribute to determine its mode of action. It 

 is, in addition, a double organ ; and impressions may be derived from 

 the simultaneous employment of both eyes, which cannot be acquired 

 by the use of one alone. Finally, the special sensibility of its nervous 

 elements is liable to modifications of various kinds, which have an in- 

 fluence upon the nature and intensity of the sensations produced. The 

 principal conditions regulating the physiological exercise of the sense 

 of sight are the following : 



Field of Vision. As the eyeball is placed in the orbit with the 

 cornea and the pupil directed forward, there is, in front of each eye, a 

 circular space within which luminous objects are perceptible ; while 

 beyond its borders, laterally and posteriori}^, nothing can be seen. 

 This space is the "field of vision." Its extreme limit, in man, reaches 

 nearly to 180 degrees of angular distance ; that is to say, with the eye 

 directed straight forward, the light from a brilliant object may be per- 

 ceived, when the object itself is placed laterally almost as far back as 

 the plane of the iris. The possibilit} r , for light which has come from 

 this direction, of penetrating the pupil and finally reaching a sensitive 

 part of the retina, depends upon the refractive power of the cornea 

 and the curvature of its anterior surface, by which the luminous ray 

 is bent inward and thus enabled to enter obliquely the orifice of the 

 pupil. In many of the lower animals, where the eyes are more promi- 

 nent than in man, and the curvatures of the cornea and crystalline lens 

 more pronounced, the field of vision is enlarged in a corresponding 

 degree. In birds and fishes, it is still further modified by the lateral 



