254 VISION. 



difficulty produced by the interposition of your head in 

 the path of entering rays. Is the image erect or inverted 1 



Again illuminate the eye from behind, with the apertures 

 open, and hold a white screen in front of the model, so 

 that the emergent rays shall fall upon it. Note that the 

 light does not come to a focus at whatever distance the screen 

 be held. The rays practically emerge parallel. In consequence 

 of this the observer's eye should have normal vision (be 

 emmetropic) and be in a state of negative accommodation, in 

 order that it may be able to focus the parallel rays on to its 

 own retina. 



Indirect method of examination (Fig. 52). 



Next mount a lens of 12 to 14 D. focus, and the mirror 

 in separate cork holders. Level them with the artificial eye. 

 Throw the light through the back of the model. Place the 

 lens 4.5 to 5 cm in front of the latter and look at its interior. 

 The apertures will be seen through the lens, but you will 

 have to place yourself some distance away (40 to 50 cm ) from 

 the model in order to see them distinctly. 



Then follow the behaviour of the light with the screen. 

 A sharp image (inverted) of the apertures in the fundus will 

 be formed at some 8 to 10 cm from the lens. The rays which 

 form this image, on being traced beyond it, diverge into 

 expanding cones, and these gaining access to your eye are 

 brought to a focus upon your retina, and there form another 

 image (erect). The latter is, however, interpreted as an 

 inverted image by the brain, and appears projected at about 

 30. cm (12 inches) from the observer. 



Close the apertures in the fundus, place the light on one 

 side of the model, and adjust the mirror so as to illuminate 

 the fundus; the black spots and the type will be seen inverted 

 and enlarged. 



