REFRACTION IN THE EYE 497 



and the lens. Examine this image through a 

 magnifying glass of + 5 D placed behind the 

 aperture of the mirror. If the observer be 

 myopic in moderate degree, the aerial image will 

 lie near his far point, and he will need no mag- 

 nifying or correcting glass; if the myopia be 

 excessive, a weak concave glass should be used. 

 If the observer be hypermetropic, the degree of 

 his hypermetropia should be added to the focal 

 distance of the magnifying glass. The confusing 

 bright reflexes from the surfaces of the 20 D lens 

 may be avoided by holding the lens slightly 

 oblique to the optical axis. 



The subject's eye and the 20 D lens form a 

 refracting system like the objective of the com- 

 pound microscope; the ophthalmoscopic lens 

 plays the part of the ocular. 



The image is real, inverted, and magnified. 

 But it will appear to be upright. In it all the 

 relations of the retinal objects are reversed. If 

 the observer move, the image will move in the 

 opposite direction. The size of the image is found 

 by formula 5, p. 61. B is the size of the aerial 

 image, b the size of the optic disk = 1.5 mm., gi 

 the focal distance of the 20 D lens == 50 mm., g 2 

 the distance from the nodal point to the retina 

 = 15 mm. Then 



B : 1.5 : : 50 : 15, and B = 5 mm. 



32 



