ALTERNATION IN ACTION OF THE EYES. 173 



that we never fix the eyes except on a very limited portion 

 of the space spread out before us; the objects thus normally 

 seen occupy our whole attention, and turn it away from the 

 other images of which the vagueness or duplication passes 

 unperceived. When we endeavour to determine these facts 

 we find that the boundary lines of the objects and the borders 

 of the pictures appear double, but dim and confused, when 

 outside of the point of convergence of the ocular axes. 



Alternation in the action of the eyes. When we look at two 

 circles in the stereoscope, alike in size but of different colours, 

 or if they are traced on white paper, and contain two different 

 letters, we distinguish alternately one image and then the 

 other, and when after a longer or shorter time we succeed in 

 seeing them superposed, very soon they again alternate. The 

 two eyes do not act simultaneously in experiments of this 

 nature, and it is sometimes the impression produced on the 

 right eye, and sometimes that of the left, alone which reaches 

 the brain. This periodicity is especially regular in persons 

 whose sight has the same range in both eyes. And we 

 remark also that the distinct image is covered with spots of 

 the same colour as that which is invisible. 



This last phenomenon seems to indicate that the retina is 

 not equally sensitive throughout its whole extent. The alter- 

 native preponderance of one eye over the other in vision is 

 due to causes not thoroughly known, though it may be 

 attributed, partially at least, to the fact that the eyes are 

 . unequal in extent of vision, or rather in skill in seeing. We 

 ostly all of us use one eye more than the other in ordinary 

 vision, and especially when we look attentively at an object. 

 It is with the eyes nearly the same as with the hands in this 

 respect, one is exercised more than the other, and it is 

 generally the right eye. We have seen that the difference 

 between the two eyes may amount to myopia in one while 

 the other is perfectly normal. This inequality, even if slight, 

 must tend to a difference in the power of accommodation 

 and to discord in action which constantly inclines to cease 

 and then to again reproduce itself. 



As for the inequality in sensibility of the different portions 

 of the retina outside of the blind point (punctum ccecum) the 



