13 MICROSCOPE AND ACCESSORIES. 



found (§ 4). Measure the distance from the paper object on the stage 

 to the objective, and it will represent approximately the principal focal 

 distance (PI. I, Fig. 1). 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 § 4). 



Aerial Image. — After seeing the real image on the ground-glass, or 

 paper, use the Japanese paper over about half of the opening of the 

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

 scope as before and shade the top of the tube by holding the hand be- 

 tween 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 ex- 

 tends 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 diffi- 

 cult 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 sur- 

 face of the glass and the delicate fibers of the paper reflect the rays ir- 

 regularly, so that the image may be seen at almost any angle, as if the 

 letters were actually printed on the paper or glass. 



The function of an objective, as seen from these experiments, 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. 5). 



FUNCTION OP AN OCULAR. 



§ 35- Using the same objective as for § 34, get as clear an image of 

 the letters as possible on the Japanese paper screen. Look at the 

 image with a simple microscope (Fig. 8 or 9) 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 (§ 4). 

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



Put an A, No. 1, 2 in. or 45 mm. ocular (i. c., an ocular of low mag- 

 nification) in position (§ 31). Hold the eye about 10 to 20 millimeters 

 from the eye-lens and look into the microscope. The letters will ap- 



