THE SENSE OF VISION (537 



end either towards or from the observer; especially if one eye 

 be closed an actual vessel can equally be "seen" in either posi- 

 tion. 



Apparatus used in Clinical Investigation of the Eye. 



For acuity of vision, Snellen's test-types are generally used (Fig. 366). 

 The size of the letters is so arranged that a person with average acuity 

 is able to read the top letter at a distance of 60 metres, the second 



T 



D L N 



P T E R 



F Z B D E 



O E L Z T C 



L F O R r E Z 



Kia. 3ti6. SSELI.EX'S TEST-TYPES KEDT:CED IN SIZE. 



line at 36 metres, the third at 24 metres, the fourth at 18 metres, 

 the smallest at 6 metres. In testing the acuity, the subject stands 



