MICROSCOPE AND ACCESSORIES 



[CH. I 



dotted tines. Ray (/) traverses the center of the lens, and is therefore not deflected. 

 It is a secondary axis {\ 5) . 



\ 10. Geometrical Construction of Images. — As shown in Figs. 14-15, for the 

 determination of any point of an image, or the image being known, to determine 

 the corresponding part of the object, it is necessary to know the position of the 

 principal focus (and there is one on each side of the lens, § 6), and the optical 

 center (Figs. 1-9 of the lens). Then a secondary axis (2) in Fig. 14, (1) in Fig. 

 15, is drawn from the extremity of the object and prolonged indefinitely above the 

 lens, or below it for virtual images. A second line is drawn from the extremity of 

 the object, (3) in Fig. 14, (2) in Fig. 15, to the lens parallel with the principal 

 axis. After traversing the lens it must be drawn through the principal focal point. 

 If now it is prolonged it will cross the secondary axis above the lens for a real 

 image and below for a virtual image. The crossing point of these lines determines 

 the position of the corresponding part of the image. Commencing with any point 

 of the object the corresponding point of the image may be determined as just 

 described, and conversely commencing with the image, corresponding points of 

 the object may be determined. 



SIMPLE MICROSCOPE : EXPERIMENTS 



§H 



Employ a tripod or other simple microscope, and for object 

 a printed page. Hold the eye about two centimeters from the upper 

 surface of the magnifier, then alternately raise and lower the magnifier 

 until a clear image may be seen. (This mutual arrangement of micro- 

 scope and object so that a clear image is seen, is called focusing). 

 When a clear image is seen, note that the letters appear as with the 

 unaided eye except that they are larger, and the letters appear erect or 

 right side up, instead of being inverted, as with the compound 

 microscope (§ 12). 



Fig. 16. Diagram of the simple microscope show- 

 ing the course of the rays and all the images, and 

 that the eye forms an integral part of it. 



A 1 B l . The object within the principal focus. A 3 

 B 3 . The virtual image on the same side of the lens 

 as the object. It is indicated with 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 retinal 

 image in the field 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. 



