CH.I] 



MICROSCOPE AND ACCESSORIES 



the magnifier until a clear image • may be seen. (This mutual 

 arrangement of microscope and object so that a clear image is seen, 

 is called focusing.) When a clear image is seen, note that the let- 

 ters 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 (§15, Fig. 15).' 



Fig. 16. Diagram of the simple micro- 

 scope showing the course of the rays and all 

 the images, and that the eye forms an integral 

 part of it. 



A z B 1 . The object within the principal 

 focus. A$ i?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*B Z ). 

 The virtual image is simply a projection of the 

 retinal image in the field of vision. 



Axis. The principal optic axis of the 

 microscope and of the eye. Cr. Cornea of the 

 eye. Li Crystalline lens of the eye. R. Ideal 

 refracting surface at which all the refractions 

 of the eye may be assumed to take place. 



§ 13. Obtaining the Principal Focus. — Hold the simple 

 microscope directly toward the sun and move it away from and 

 toward a piece of printed paper until the smallest bright point is 

 obtained. This is the burning point or focus and as the rays of the 

 sun are nearly parallel, the burning point represents approximately 

 the principal focus (Fig. n). The above and following operations 

 are more easily accomplished if the lens is supported as in Fig. 22. 



§ 14. Real and Virtual Images with a Simple Micro- 

 scope. — Without changing the position of the magnifier or paper 

 look into the magnifier, holding the eye close to the upper surface 

 and the letters on the paper may be seen, but they will appear much 

 sharper to the eyes o'f most people if the magnifier is brought nearer 

 to the paper, that is so that the printed paper is within the principal 

 focal distance (Fig. 15 and 16). 



After getting as clear an image as possible by focusing the 

 simple microscope, raise the magnifier until the letters are at a dis- 



