THE EYE. 



eyes bear an insignificant proportion to the distance of the object 

 from them, the lines drawn from the centre of the eyes to any 

 point on the object will be practically parallel ; and. since these 

 lines, as has been already explained, determine the direction in 

 which the object is seen, such object will then be seen in the same 

 direction. Now, since the apparent form, the apparent magnitude, 

 the apparent colour, the apparent intensity of illumination, 

 and the apparent direction are the same for both eyes, it is 

 clear that the same impression must be produced upon the senses, 

 and the same perceptions conveyed to the mind ; consequently it 

 follows, demonstratively, that all objects which are placed at a 

 distance compared with which the distance between the eyes : s 

 insignificant, will convey a single perception to the mind, and 

 will consequently not be seen double. 



77. But we have now to consider a different case, which will 

 present peculiar conditions, and consequences of peculiar interest. 



Let us suppose an object placed so near the eyes that its dis- 

 tance shall not bear a considerable proportion to the length of the 

 line which separates the centres of the eyes. In this case, the 

 images produced on the retina of the two eyes may differ in 

 magnitude, and intensity of illumination, and even in form, and, 

 in fine, it is clear that the apparent direction of any point on the 

 object as seen by the two eyes will be sensibly different. 



In this case, therefore, the two eyes convey to the mind a dif- 

 ferent impression of the same object ; and we may therefore expect 

 that we should see it double, and in fact we do so. 



But the observation of this particular phenomenon requires 

 much attention, inasmuch as the perception of which we are con- 

 scious is affected not merely by the impression made upon the 

 organ of sense, but by the degree of attention which the mind 

 gives to it. Thus, if the two eyes be differently impressed either 

 by the same or by different objects placed before them, the mind 

 may give its attention so exclusively to either impression, as to lose 

 all consciousness of the other. 



Thus, if two stars be at the same time in the field of view of a 

 telescope, as frequently happens, and be viewed together by lie 

 eye, we shall be conscious of a perception of both, so long as the 

 attention is not exclusively directed to either; but if we gaze 

 intently on one of them so as to observe its colour, or any other 

 peculiarity attending it, we shall cease to be conscious of the 

 presence of the other. The application of this observation to the 

 question before us will be presently apparent. 



Let EL, fig. 12, be the line separating the centres of the two 



eyes, u representing the centre of the right, and L that of the left 



eye. 



84 



