CH. I] MICROSCOPE AND ACCESSORIES 31 



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 fibres of the paper reflect the rays irregularly, 

 so that the image ma}- be seen at almost any angle, as if the letters 

 were actually printed on the paper or glass. 



§ 55. The Function of an Objective, as seen from these experi- 

 ments, 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. 21 ). 



FUNCTION OF AN OCULAR 



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

 of the letters as possible on the lens paper screen. Look at the image 

 with a simple microscope (Fig. 17 or 18) 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, that is they appear as with the naked eye (§ n). 

 Remove the screen and observe the aerial image with the tripod. 



Put a 50 mm. (A, No. 1 or 2 in.), ocular i. <?., an ocular of low 

 magnification) in position (§ 48). Hold the eye about 10 to 20 milli- 

 meters 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. 



§ 57. The Function of the Ocular, as seen from the above, is 

 that of a simple microscope, viz. : It magnifies the real image formed 

 by the objective as if that image were an object. Compare the image 

 formed by the ocular (Fig. 21), and that formed by a simple microscope 

 (Fig. 3 8). 



It should be borne in mind, however, that the rays from an object 

 as usually examined with a simple microscope, extend from the object 

 in all directions, and no matter at what angle the simple microscope is 

 held, provided it is sufficiently near and points toward the object, an 

 image may be seen. The rays from a real image, however, are continued 

 in certain definite lines and not in all directions ; hence, in order 

 to see this aerial image with an ocular or simple microscope, or 



