30 MICROSCOPE AND ACCESSORIES \_CH. I 



Lower the tube of the microscope by means of the coarse adjust- 

 ment until the objective is within 2-3 cm. of the object on the stage. 

 Look at the screen on the top of the tube, holding the head about as 

 far from it as for ordinary reading, and slowly elevate the tube by means 

 of the coarse adjustment until the image of the letter appears on the 

 screen. 



The image can be more clearly seen if the object is in a strong 

 light and the screen in a moderate light, i. e., if the top of the micro- 

 scope is shaded. 



The letters will appear as if printed on the ground glass or paper, 

 but will be inverted (Fig. 21). 



If the objective is not raised sufficiently, and the head is held too 

 near the microscope, the objective will act as a simple microscope. If 

 the letters are erect, and appear to be down in the microscope and not 

 on the screen, hold the head farther from it, shade the screen, and 

 raise the tube of the microscope until the letters do appear 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 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 distance (Figs. 10, 11). 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 § 11). 



§ 54. 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 



