206 'ANSWERS TO PRACTICAL QUESTIONS. 



second proceeds from the front of the crystalline lens, which 

 has the same effect ; while the inverted image proceeds from 

 the posterior face of the lens, which, being convex backward, 

 is, of course, concave forward, and acts as a concave mir- 

 ror. HUXLEY. The images formed upon the retina are as 

 fleeting as light itself, from the nature of the case, and dis- 

 appear as soon as the object is removed. 



