1 4 MICROSCOPE AND ACCESSORIES. 



pear as when the simple microscope was used (see above), the image 

 will become more distinct by slightly raising the body of the micro- 

 scope with the coarse adjustment. 



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 ob- 

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

 by the ocular (Fig. 5), and that formed by a simple microscope (Fig. 4). 



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 con- 

 tinued in certain definite lines and not in all directions ; hence, in 

 order to see the 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. 



The field-lens of a Huygenian ocular makes the real image smaller 

 and consequently increases the size of the field ; it also makes the im- 

 age brighter by contracting the area of the real image (PI. I, Fig. 6). 

 Demonstrate this by screwing off the field-lens and using the eye-lens 

 alone as in the ocular, refocusing if necessary. Note also that the let- 

 ters or other image is bordered by a colored haze (§ 22). 



When looking into the ocular with the field-lens removed, the eye 

 should not be held so close to the ocular, as the eye-point is consider- 

 ably farther away than when the field-lens is in place (PI. I, Fig. 7, 

 and § 36). 



§ 36. The Eye- Point. — This is the point above an ocular or simple 

 microscope where the greatest number of emerging rays cross. Seen 

 in profile, it may be likened to the narrowest part of an hour-glass. 

 Seen in section (PI. I, Fig. 7), it is the smallest and brightest light 

 circle above the ocular. This is called the eye-point, for if the pupil 

 of the eye is placed at this level, it will receive the greatest number of 

 rays from the microscope, and consequently see the largest field. 



Demonstrate the eye-point by having in position an objective and 

 ocular as above (§ 35). Ljght the object brightly, focus the micro- 

 scope, shade the ocular, then hold some ground-glass or a piece of 

 the Japanese paper (§ 72) above the ocular and slowly raise and lower 

 it until the smallest circle of light is found. By using different oculars 

 it will be seen that the eye-point is nearer the eye-lens in low than in 

 high oculars. 



