THE DOUBLE CONVEX LENS. 279 



focus, the amount of heat can be appreciated. The well-known 

 meridian gun in the Palais Royal is fired by the sun. A burn- 

 ing-glass of no very great size is placed over the touch -hole of 

 the gun, with which its focus coincides. The lens is turned 

 in such a manner that, as the sun attains the meridian, its 

 rays are thrown upon the touch-hole, and consequently fire 

 the gun. 



The word focus is the Latin term for a domestic hearth, and 

 is used in allusion to the heat which is manifested at the 

 point on which the rays of the sun converge. 



It is evident that, after reaching the focus, the rays, if they 

 be not intercepted by some object, will cross each other, and 

 form a large image, but reversed. This part of the subject 

 will presently be explained. 



The accompanying illustration shows two figures, one repre- 

 senting the section of a double convex lens made by the hands 

 of man, and the other that of a double convex lens as seen in 

 Nature. 



CRYSTALLINE LENS OF HUMAN EYE. DOUBLE CONVEX LENS. 



The former has already been explained. The latter is the 

 double convex lens of the human eye, by means of which the 

 images of external objects are conveyed to the brain. When- 

 ever this lens becomes thickened by disease, the sight is 

 gradually dimmed, and at last total blindness is the result. 

 This disease is popularly called " cataract," and until late days 

 was incurable. JSTow, however, any good oculist will attack a 

 cataract, and either partially or entirely restore the sight. This 

 operation is performed by carefully removing the convex lens, 

 and supplying its place with a glass lens, which throws the 

 rays of light on the same focus. 



