CH. /.] 



MICROSCOPE AND ACCESSORIES. 



31 



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 (§9). Remove the 

 screen and observe the aerial image with the tripod. 



Put a 50 mm., (A, No. 1, or 2 in.) ocular (z. e., an ocular of low 

 magnification) in position (§ 44). Hold the eye about 10 to 20 milli- 

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

 will appear as when the simple microscope was used (see above), the 

 image will become more distinct by slightly raising the tube of the mi- 

 croscope with the coarse adjustment. 



§ 53. The Function of the Ocular, as seen from the above, is that 

 of a simple microscope, viz. : It magnifies the real image formed by the 

 objective as if that image were an object. Compare the image formed 

 by the ocular (Fig. 21), and that formed by a simple microscope (Fig. 



38). 



It should be borne in mind, however, that the rays from an object as 

 usually examined with a simple microscope, extend from the object in 

 all directions, and no matter at what angle the simple microscope is held, 

 provided it is sufficiently near and points toward the object, an image 

 may be seen. The rays from a real image, however, are continued in 



Fig. 38. Diagram of the simple microscope show- 

 ing the course of the rays and all the images, and 

 that the eye forms an integral part of it. 



A 1 B l . The object within the principal focus. A 3 

 B 3 . The virtual image on the same side of the lens 

 as the object. It is indicated with dotted lines, as 

 it has no actual existence. 



B' A ! . Retinal image of the object (A 1 B l ). The 

 virtual image is simply a projection of the retinal 

 image in the field of vision. 



Axis. The principal optic axis of the microscope 

 and of the eye. Cr. Cornea of the eye. L. Crystal- 

 line lens of the eye. R. Ideal refracting stirf ace at 

 which all the refractions of the eye may be assumed 

 to take place. 



certain definite lines and not in all directions ; 



hence, in order to see this aerial image with 



an ocular or simple microscope, or in order 



to see the aerial image with the unaided eye, the simple microscope, 



ocular or eye must be put in the path of the rays (Fig. 21). 



§ 54. The field-lens of a Huygenian ocular makes the real image 

 smaller and consequently increases the size of the field ; it also makes 

 the image brighter by contracting the area of the real image (Fig. 30). 



AK 



