238 LAB OR A TOR Y G UIDE IN PHYSIOLOG Y. 



III. To test the color sense. 



Let the subject take the three test colors: light 



green, purple and red, and choose from the mass of 



worsteds the colors which he considers similar ones, 



placing the chosen color in the class to which it be- 



longs. It is not difficult to determine whether or not 



the subject has a normal color sense. If, for example, 



he is red blind he will not see the red in the purple, 



or related colors, but will classify these with the blues, 



while the reds will be confused with the greens. 



b. The range of accommodation __ The amount of 



refractive change induced by the eye in adjusting for its 



punctumproximum after it has been at rest, i. e., after it 



has been adjusted for its punctum remotum, is termed 



the range of accommodation. In a previous chapter 



the punctum proximum and punctum remotum were deter- 



mined. It was reserved for this place to express the 



position of these limits of accommodation in terms of 



dioptres, and thus most readily determine and definitely 



express the range in simple dioptres. The relation of 



this to what has just preceded will be evident. 



Let R represent the distance of the punctum remotum 

 from the eye, then the refraction at rest or the static re- 

 fraction r equals the reciprocal of the distance: 



Let P be the distance of the punctum proximum from 

 the eye, then the maximum refraction of the eye, p 

 equals the reciprocal of the distance: 



(2) p = |. 



When R = oo, - = 0, i. e., static refraction equal zero. 

 When P =/s meter, -~ 8. 



