REFRACTION IN THE EYE 467 



this angle. Only such letters are used as can 

 be drawn approximately within a square that 

 shall contain twenty -five of the smaller squares, 

 and shall subtend an angle of 5'. Thus the 

 strokes and, so far as possible, the spaces between:: 

 the strokes 'are one fifth the size of the letter.; 

 The size of the letter the perception of which 

 constitutes normal vision at a given distance 

 ^that is, the letter that subtends a visual angle 

 of 5' at the given distance) is obtained by multi- 

 plying the distance by ,0.00 1454 mm., which is 

 the tangent 1 of the angle of 5'. At the distance 

 of one metre the size of the standard letter is 

 1000 X 0.001454 = 1.45 mm. Near each of Snel- 

 len's test letters is recorded the distance viewed 

 from which the letter will subtend a visual angle 

 of 5' in the emmetropic eye. 



As some of the letters are not easily recognized 

 by the astigmatic eye (D, for example, being some- 

 times mistaken for B), the acuteness of vision 

 should not be pronounced normal unless each 

 letter of the entire series can be read at? the dis- 



1 To obtain the tangent of an angle draw a circle with the 

 vertex of the angle as the centre. The two legs of the angle 

 are radii of the circle. Diaw a perpendicular (tangent line) 

 from the end of one radius to the prolongation of the other. 

 Divide the length of the perpendicular by the length of the 

 radius ; the quotient is the function called the tangent of the 

 angle 



