32 FUNCTION OF AN OCULAR [Ch. I 



in the air. Another striking experiment is to have a small hole in 

 the paper placed over the center of the tube opening, then if a printed 

 word extends entirely across the diameter of the tube, its central 

 part may be seen in the air, the lateral parts on the paper. The ad- 

 vantage of the paper over part of the opening is to enable one to accom- 

 modate the eyes for the right distance. If the paper is absent the eyes 

 adjust themselves for the light circle at the back of the objective, and 

 the aerial image appears low in the tube. Furthermore it is more 

 difficult to see the aerial image in space than to see the image on the 

 ground-glass or paper, for the eye must be held in the right position 

 to receive the rays projected from the real image, while the granular 

 surface of the glass and the delicate fibers of the paper reflect the rays 

 irregularly, so that the image may be seen at almost any angle, as if 

 the letters were actually printed on the paper or glass. 



§ 52. The function of an objective, as seen from these experiments, 

 is to form an enlarged, inverted, real image of an object, this image 

 being formed on the opposite side of the objective from the object 

 (fig. 13, 20). 



Function of an Ocular 



§ 53. Using the same objective as for •§ 50, get as clear an image 

 of the letters as possible on the lens paper or ground-glass screen. 

 Look at the image with a simple microscope (fig. 17), as if the image 

 were an object. 



Observe that the image seen through the simple microscope is 

 merely an enlargement of the one on the screen, and that the letters 

 remain inverted. Remove the screen and observe the aerial image 

 with the tripod magnifier. 



§ 54. Put a 4X or 5X ocular, i.e., an ocular of low magnification in 

 position (§ 45). Hold the eye about 10 to 20 mm. from the eye-lens 

 and look into the microscope. The letters will appear as when the 

 simple microscope was used (see above) ; the image will become more 

 distinct by slightly raising the tube of the microscope with the coarse 

 adjustment. 



§ 55. The function of the ocular, as seen from the above, is that 

 of a simple microscope, viz. it magnifies the real image formed by 



