1911.] The Causes and Prevention of Miners' Nystagmus. 



21 



I have tested my cases for visual acuity and error of refraction ; it was 



impossible to examine all, from lack of opportunity or time. As it was 

 important not to call the attention of the latent cases to the fact that they 

 had nystagmus these were not examined. 



Table XVII.— Visual Acuity. 



Too bad to test 18 



Not examined 39 



Normal 43 



6/9 53 



6/12 46 



6/18 67 



6/60 39 



The fraction gives the proportion of power of vision to normal; thus 

 6/12 = half normal, etc. 



Table X VIII.— Showing Error of Eefraction. 



Not examined 46 



Latent 43 



Too bad to test 31 



Apparently normal 18 (6 - 4 per cent.) 



Error 262 (93"6 per cent.) 



Table XIX.— Showing Kind of Error. 



Hyper me tropia (long sight) 105 



Myopia (short sight) 37 



Astigmatism — 



Hypermetropic 91 



Myopic 26 



Mixed 3 



Table XX. — Showing Degree of Error. 



Less than 1 Diopter 102 (chiefly astigmatic) 



lto2D 110 



2 to 3 30 



3 to 4 9 



4 and over 11 



All have weakness of accommodation. They are unable to fix a point 

 for any length of time, and they turn away their heads or cover their eyes 

 with their hands after a short time. 



Many cases show internal squint, and injection of the eyes is frequent. 



