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 alternative 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 almostly 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 cúcum) the displacement of the spots proves that it is not permanent We know also that this partial insensibility may be induced by a brilliant light, and particularly by the rays of the sun. It is an experiment we all make involuntarily, and which will be discussed in another place.