30 MICROSCOPE AND ACCESSORIES. [CH. 



To demonstrate that the object must be outside the principal foci 

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

 the microscope directly toward the sun. Move the tube of the mien 

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

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

 objective, and it will represent approximately the principal focal dii 

 tance (Figs. 10, n). Replace the screen over the top of the tube, n 

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

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

 objective is now taken, it will bs found considerably greater than th 

 principal focal distance (compare § 9). 



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

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

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

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

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

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

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

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

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

 extends entirely across the diameter of the tube its central part may b 

 seen in the air, the lateral parts on the paper. The advantage of th 

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

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

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

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

 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 receiv 

 the rays projected from the real image, while the granular surface 

 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 letter 

 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, thi 

 image being formed on the opposite side of the objective from the ob 

 ject (Fig. 21). 



FUNCTION OF AN OCULAR. 



§ 52. Using the same objective as for § 49, get as clear an image o 

 the letters as possible on the lens paper screen. Look at the imagi 

 with a simple microscope (Fig. 17 or 18) as if the image were an object 



