466 THE OUTGO OF ENERGY 



the extremities of , these arrows to the nodal 

 point. 



Observe that as the object recedes from the 

 eye the visual angle and the retinal image become 

 smaller. When the visual angle is less than 

 one minute, the retinal image will be too small 

 to be perceived ; the limit of perception will be 

 reached. ,-. ... ' 



Smallest Perceptible Image. On a black card 

 gum one millimetre apart, and parallel with each 

 other, two slips of white paper one millimetre 

 in width. Place the card about six metres in 

 front of a window or other sufficient light. Face 

 the card and move backward until the millimetre 

 space between the two white slips disappears be- 

 cause the slips can no longer be seen separately. 

 Measure the distance gi from the object to the 

 nodal point. Calculate the size of the retinal 

 image ( formula 5). Compare this result with 

 the diameter of the cones in the region of dis- 

 tinct vision (page 461). 



Measurement of Visual Acuteness. Taking I/ 

 as the average smallest visual angle at which an 

 object is perceptible, Snellen built up a set of 

 test letters by combining small squares each of 

 which subtends an angle of 1'. Thus the lines 

 of which the letters are formed subtend an angle 

 of 1'. The spaces between the lines also subtend 



