30 MICROSCOPE AND ACCESSORIES. [CM. I. 



To demonstrate that the object must be outside the principal focus 

 with the compound microscope, remove the screen and turn the tube of 

 the microscope directly toward the sun. Move the tube of the micro- 

 scope with the coarse adjustment until the burning or focal point is found 

 (§ 6). Measure the distance from the paper object on the stage to the 

 objective, and it will represent approximately the principal focal dis- 

 tance (Figs. 10, ii). Replace the screen over the top of the tube, no 

 image can be seen. Slowly raise the tube of the microscope and the 

 image will finally appear. If the distance between the object and the 

 objective is now taken, it will be found considerably greater than the 

 principal focal distance (compare § 9). 



§ 50. Aerial Image. — After seeing the real image on the ground- 

 glass, or paper, use the lens paper over about half of the opening of 

 the tube of the microscope. Hold the eye about 250 mm. from the 

 microscope as before and shade the top of the tube by holding the hand 

 between it and the light, or in some other way. The real image can 

 be seen in part as if on the paper and in part in the air. Move the 

 paper so that the image of half a letter will be on the paper and half 

 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 advantage of the 

 paper over part of the opening is to enable one to accommodate 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. 



§ 51. 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 ob- 

 ject (Fig. 21). 



FUNCTION OP AN OCULAR. 



§ 52. Using the same objective as for § 49, 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. 



