EXPLANATION OF PLATE I. 



In all of the figures, Virtual Images and rays traced backward or produced rays, 

 are indicated by dotted or broken lines, real rays or paths of rays by full or un- 

 broken lines. 



Fig. i. Sectional view of a double convex lens showing: (A B) the principal 

 plane at which the refractions of the curved surfaces are most conveniently shown ; 

 (c) Optical Center of the lens. Rays traversing this center undergo no deviation. 



Axis. Principal optic axis of the lens, i. e., line connecting the centers of cur- 

 vature of the two surfaces of fhe lens. The axis traverses the optical center and 

 the principal focal point or focus (F). F. Principal focal point or focus, i. e., the 

 point where central parallel rays are brought to a focus. 



F D. Principal focal distance, or focal length, i. e., the distance between the cen- 

 ter of the lens (c) and the principal focus (F). 



Fig. 2. Convex lens showing the position of the object (A B) within the princi- 

 pal focus and the course of rays in the formation of a virtual image. 



A B. The object placed between the lens and its focus; A'B' virtual image 

 formed by tracing the rays backward. It appears on the same side of the lens as 

 the object, and is erect (£4). 



Axis. The optic axis of the lens. The principal focus is represented by a dot 

 on the axis between the object and virtual image. 



1, 2, 3. Rays from the point B of the object. Thej r are diverging after travers- 

 ing the lens, but not so divergent as if no lens were present, as is shown by the 

 dotted lines. Ray 1 traverses the center of the lens, and is therefore not deviated. 



Fig. 3. Convex lens showing the position of the object (A-B) outside the princi- 

 pal focus (F), and the course of the rays in the formation of real images. To avoid 

 confusion the rays are drawn from only one point, as in Fig. 2. 



A B. . Object outside the principal focus. B' A / . Real, enlarged image on the op- 

 posite side of the lens. 



Axis. Principal optic axis. 1, 2, 3. Rays after traversing the lens. They are 

 converging, and consequently form a real image. The dotted lines and the line 2 

 give the direction of the rays unaffected by the lens. F. The principal focus. 



Fig. 4. 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 B. The object within the principal focus. A' B'. 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. 



A 2 B\ Retinal image of the object (A B). The virtual image is simply a projec- 

 tion 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. h Crystalline lens of the eye. R. Ideal refracting surface at which 

 all the refractions of the eye may be assumed to take place. 



Fig. 5. Diagram of a compound microscope, showing the course of the rays from 

 the object (A'B ) through the objective to the real image (BA), thence through 

 the ocular and into the eye to the retinal image (A- B 2 ), and the projection of the 

 retinal image into the field of vision as the virtual image (B' A')- 



A'B'. The object. A- B-. The retinal image of the inverted real image, B A, 



