474 THE OUTGO OF ENERGY" 



that meet the refracting surfaces at a distance 

 from the optical axis (compare page 43i). 



Relation of Iris to Lens. 1. Stand the convex 

 mirror upright on a level with the eye of the 

 observer. Over the mirror (Fig. 66, C M) place 

 a diaphragm of black paper, T I', with an aperture, 

 PP', four millimetres in diameter. Let this aper- 

 ture be the pupil and the convex mirror be the 

 crystalline lens. The wooden block in which the 

 mirror is held will support the diaphragm so that 

 there will be a space between the border of the 

 pupil and the surface of the mirror. Let the 

 lamp, L, be on one side of the aperture and 

 the observer's eye, E, on the other. By. means 

 of the convex lens of 6.5 cm. focal distance fur- 

 nished with the ophthalmoscope concentrate the 

 light upon the margin of the pupil in the direc- 

 tion LH. It will pass the margin P and be 

 reflected from the mirror to the eye in the direc- 

 tion H E. No rays from L can reach the mirror 

 between H and C. This portion of the mirror 

 will reflect 'the posterior side of ,the diaphragm; 

 Thus the light from S falling on the mirror at J 

 will be reflected in the direction J E to the ob- 

 server's eye, and a dark band, the image of the 

 back of the diaphragm, will appear in the mirror 

 between the image of L at H and the margin of 

 the pupil. -:.-'. 



