THE SENSE OE VISION. 



361 



Fig. 178.- 



Illusion of lines always pointing 

 toward observer. 



is drawn as it appears to an eye looking straight down the barrel, and, as this 



foreshortening is the same in all positions of the observer, it is evident that 



when such a picture is hung upon the wall 



of a room the soldier will appear to be 



aiming directly at the head of every person 



present. 



In concluding this brief survey of some 

 of the most important subjects connected 

 with the physiology of vision it is well to 

 utter a word of caution with regard to a 

 danger connected with the study of the sub- 

 ject. This danger arises in part from the 

 fact that in the scientific study of vision it 

 is often necessary to use the eyes in a way 

 quite different from that in which they are 

 habitually employed, and more likely, there- 

 fore, to cause nervous and muscular fatigue. 

 We have seen that in any given position of 

 the eye distinct definition is limited to an 

 area which bears a very small proportion to 



the whole field of vision. Hence in order to obtain an accurate idea of the 

 appearance of any large object our eyes must wander rapidly over its whole 

 surface, and we use our eyes so instinctively and unconsciously in this way 

 that, unless our attention is specially directed to the subject, we find it diffi- 

 cult to believe that the power of distinct vision is limited to such a small 

 portion of the retina. In most of the experiments in physiological optics, 

 however, this rapid change of direction of the axis of vision must be carefully 

 avoided, and the eye-muscles held immovable in tonic contraction. 



Our eyes, moreover, like most of our organs, serve us best when we do not 

 pay too much attention to the mechanism by which their results are brought 

 about. In the ordinary use of the eyes we are accustomed to neglect after- 

 images, intraocular images, and all the other imperfections of our visual appa- 

 ratus, and the usefulness of our eyes depends very much upon our ability thus 

 to neglect their defects. Now, the habit of observing and examining these 

 defects that is involved in the scientific study of the eye is found to interfere 

 with our ability to disregard them. A student of the physiology of vision 

 who devotes too much attention to the study of after-images, for instance, may 

 render his eyes so sensitive to these phenomena that they become a decided 

 obstacle to ordinary vision. 



