MICROSCOPE AND ACCESSORIES 



[CH. I 



tance a little greater than the principal focal distance. Look into 

 the magnifier and note the clearness of the virtual image, then 

 slowly elevate the head above the magnifier and when the eye is 

 about 60 to 100 centimeters above the lens a real image can be seen. 

 That is an image in which the letters are inverted as with the 

 objective of the compound microscope (see § 60). If the magnifier 

 is raised somewhat so that the printed' letters are markedly without 

 the principal focus the real image will be seen more clearly espec- 

 ially if the eye is brought somewhat near the magnifier. The above 

 experiments show two things. 



Fig. 17. Figures of a normal [emme- 

 tropic), afar sighted (hyperopic) and a 

 short sighted (myopic) eye_ to show that 

 when the eye is at rest the normal eye (E) 

 focuses parallel rays on the retina while the 

 far-sighted eye (//) focuses parallel rays 

 beyond the retina. The short sighted eye 

 (M) focuses parallel rays in front of the re- 

 tina. The dotted lines show that in the 

 hyperopic eye the rays must be converging 

 to come to a focus on the retina while with 

 the myopic eye they must be diverging. 



(1) That every convex or converging lens or lens system can 

 serve to form either a virtual or a real image, depending upon its 

 position with reference to the object. 



(2) They show also that without changing the position of the 

 magnifier, if it is slightly further from the object than its principal 

 focal distance, either a virtual image or a real image may be seen by 

 many people, depending upon the position of the eye. (a) If the 

 eye is close to the magnifier an enlarged erect virtual image will be 

 seen, (b) With the eye at a considerable distance an enlarged 

 inverted real image may be seen. 



While the law is absolute that real images are formed only 

 when the object is without the principal focal distance, and virtual 

 images only when the object is within the focus, the above experi- 

 ments show most conclusively that the eye is a part of the optical 



