REFRACTION IN THE EYE 481 



it shall subtend the standard angle of 5' when 

 placed 25 cm. from the cornea. The distance from 

 the eye at which this type can be read clearly 

 = P. 



Range of Accommodation. The range of accom- 

 modation is the expression of the total accommoda- 

 tive power of the eye. With the eye at rest rays 

 diverging from the far point r are brought to a 

 focus on the retina, With the ciliary muscle 

 fully contracted, rays diverging from the near 

 point p are focussed on the retina. To bring the 

 more divergent rays from p to the same focal 

 plane as the less divergent rays from r, an auxiliary 

 lens must be employed, as in the following ex- 

 periment. 



Place the diaphragm with |_-shaped aperture 

 in front of the condenser. Eemove the tubes 

 holding the projecting lenses. Kays will now 

 diverge from the illuminated I Place this illu- 

 minated object at a convenient far point, for ex- 

 ample, 26 cm. in front of the convex lens of 10 crn. 

 focal length. Place a screen at the conjugate 

 focus. Note the clear image. Move the object 8 cm. 

 nearer the lens. Let this be the near point. The 

 conjugate focus now falls behind the screen and 

 the image is blurred by dispersion circles. Place 

 in front of the 10 D lens an auxiliary lens of + 2 

 D. The image will be clear. The rays diverg- 



31 



