1916] 



on Eyesight and the War 



693 



We now pass to the important subject of eyestrain as it affects 

 our soldiers. 



We have seen that the eye may vary from the normal by being 

 too long or too short, and combined with these defects we may have 

 another defect, viz. astigmatism. This error lies chiefly in the 

 cornea, and is due to its surface not being spherical. In regular 

 astigmatism any one curve of the cornea is different from any other 

 curve, the maximum difference being between two curves at right 

 angles to each other. Each curve has its separate focal point, so 

 that rays of light passing through a regular astigmatic surface focus 

 in a line of points instead of one point. 



2500 

 Individuals whose sight 

 after correction was 

 normal and who had 

 no disease of the 

 eyes. 



5000 eyes (as above)— 

 Emmetropia 

 Hypermetropia 

 Myopia 

 Astigmatism 



( (1) Same refrac- 



(a) Emmetropia (see 

 Presbyopia below) 



tioninboth-< (b) Hypermetropia 



eyes (657) 



(c) Myopia 



(d) Astigmatism — 



Hypermetropic 



Myopic 



Mixed 



9 

 63 

 22 



438 

 113 



19 



(2) Refraction different in the two eyes 

 "^ (Anisometropia) .... 1843 



2500 



56 



425 



216 



4303 



5000 



Of the 2500 individuals, 961 were presbyopic, and only 9 of these 

 were emmetropic. 



By including low errors of astigmatism (which we now know are 

 the most important) we find that this is a much more common defect 

 in the eye than people imagine. In the 5000 eyes of the 2500 

 individuals that I carefully examined, you will see by the above table 

 that no less than 4308 were astigmatic. That, probably, is a higher 

 average than would have been the case if those 2500 individuals 

 were taken promiscuously. They were all people that were sent to 

 me because there was a suspicion about their eyes. But it is not a 

 very great exaggeration, because I include in that low errors of 

 astigmatism which we shall soon see are most important. 



Regular astigmatism is cured by cylindrical glasses. When rays 

 pass through a cylinder they are not refracted along the line of the 

 axis of the cylinder, but are more and more refracted as they pass 



