468 THE OUTGO OF ENERGY 



tance corresponding to the number of the series. 

 The acuteness of vision is expressed by ; where 



d is the greatest distance at which the letters 

 in any line are seen distinctly by the eye exam- 

 ined, and D the distance at which they can be 

 seen by the normally acute eye. 



Place the subject in a well-lighted room six 

 metres (approximately 20 feet) in front of a 

 card of Snellen's test types. Kays from an object 

 six metres distant are practically parallel. At 



this distance the letters numbered VI should 



/ 



be read. If they are clearly visible, V = - ; acute- 



b 



ness of vision is normal. If the subject at 6 metres 

 cannot see distinctly letters larger than those 

 marked XVIII metres (approximately 60 feet), 



s* 



V = ; acuteness of vision is one third the 



lo 



normal. 



Jn some eyes vision is so acute that types 

 constructed with a visual angle of 4 minutes 

 (| the normal angle) can be seen clearly. 



