CH. /] MICROSCOPE AND ACCESSORIES 35 



screen, and raise the tube of the microscope until the letters do ap- 

 pear on the ground glass. 



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 directty toward the sun. Move the tube 

 of the microscope with the coarse adjustment until the burning or 

 focal point is found (7,13). Measure the distance from the paper 

 object on the stage to the objective, and it will represent approx- 

 imately the principal focal distance (Figs. 10, n). 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 that the principal focal distance 

 (compare 12). 



62 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 accomodate 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 deli- 

 cate fibres 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. 



63 The Function of an Objective, as seen from these ex- 

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



