CH. /] MICROSCOPE AND ACCESSORIES 37 



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 direc- 

 tions; 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 in the path of the rays 

 (Fig. 26). 



66. 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. 36.) 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 (8). 



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 con- 

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



67. The eye-point. This is the point above the 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 (Fig. 36), 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 ( 60). Light the object brightly, focus the 

 microscope, shade the ocular, then hold some ground-glass or a 

 piece of the lens paper 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 

 high than in low oculars, that is the eye-point is nearer the eye-lens 

 for an ocular of small equivalent focus than for one of greater focal 

 length. 



* The bright circle above the ocular is sometimes called the Ramsden 

 Circle or Disc. See Carpenter-Dallinger, p. 106; Spitta, 114-118; Wright p. 

 157 ; Beck, p. 14. 



