THE SENSE ORGANS 



187 



that the pin appears to enter the visual field from above. Draw a 

 diagram to illustrate the course of the rays and explain this peculiar 

 phenomenon. 



8. The Emmetropic Eye. Adjust the optical box at a distance of 

 about 15 cm. from the optical lantern. Obtain parallel rays by placing 

 the tin tube containing the projection lenses in front of the incandescent 

 light. Lessen this bundle of light by a diaphragm, bearing the outlines 



FIG. 110. OPTICAL LANTERN AND Box. (Harvard Apparatus Co.) 



of the letter L. Close the round opening in the optical box by a flat 

 piece of window glass (cornea). In the holder directly behind this 

 opening place a convex lens of 10 cm. focal distance (10 diopters). 

 Arrange the solid black screen (retina) near the center of the optical 

 box. Close the box with a large plate of clean glass and burn a small 

 amount of Japanese incense in the box. Move the optical box as well as 

 the screen (retina) until a perfectly clear image of object L is obtained 

 upon the latter. 



