Ch. I] EYE-POINT OF AN OCULAR 33 



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

 formed by the ocular (fig. 3, 20) and that formed by a simple micro- 

 scope (rig. 2, 6). 



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 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 micro- 

 scope, ocular, or eye must be in the path of the rays (fig. 2-3). 



§ 56. 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. 23, 

 24). Demonstrate this by screwing off the field-lens and using 

 the eye-lens alone as an ocular, refocusing if necessary. Note that 

 the image is bordered by a colored haze (Ch. IX). 



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 (fig. 23) 

 is considerably farther away than when the field-lens is in place. 



§ 57. Eye-point. — This is in the plane above the ocular where 

 the emerging rays cross (fig. 22-24). If the eye is placed at this 

 point it will receive the greatest number of rays from the microscope 

 and thus see the largest field. If the eye is too far from or too near 

 the ocular, part of the rays cannot enter the pupil of the eye and the 

 microscopic image is restricted. 



Demonstrate the eye-point by using a 16 mm. objective and a 4X 

 or 5X ocular. Light brightly and then focus the microscope on some 

 transparent specimen. Open the diaphragm widely so that the entire 

 aperture of the objective is filled with light (fig. 45). Shade the 

 ocular with the hand or a screen and hold above the eye-lens a piece 

 of ground-glass or of the lens paper. By raising and lowering the 

 glass or paper one will find the level where the sharpest and brightest 

 light circle is located. The height varies with different oculars. Now 

 use the tripod or other magnifier and look at the eye-point. It is 



