3 2 



MICROSCOPE AND ACCESSORIES 



[CH. I 



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. 21.) 



Fig. 38. Diagram of the simple microscope 

 showing the course of the rays and all the images, 

 and that the eye forms an integral part of it. 



A 1 B\ The object within the principal focus. 

 A* B 3 . The virtual image on the same side of 

 the lens as the object. It is indicated by dotted 

 lines, as it has no actual existence. 



B- A-. Retinal image of the object (A 1 B 1 ). 

 The virtual image is simply a projection of the 

 retina! image into the f eld of vision. 



Axis. The principal optic axis of the micro- 

 scope and of the eye. Cr. Cornea of the eye. L. 

 Crystalline lens of the eye. R. Ideal refracting 

 surface at which all the refractions of the eye may 

 be assumed to take place. 



A'K 



f^%B> 



§ 5S. 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.) Demonstrate this by screwing off the field-lens and using 

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

 the image is bordered by a colored haze (§7). 



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. 



§ 59. 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. 30), 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 (§ 53). Light the object brightly, focus the micro- 

 scope, 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 



