188 



CONVERGENCE OF EAYS. 



great thickness of glass in the central parts, all of which 

 thickness would be useless in 

 itself, since the light, in pass- 

 ing through, is changed in its 

 direction only at the surfaces 

 where it enters and where it 

 emerges. It undergoes no 

 change of direction while it is 

 passing through the substance 

 of the glass within. In other 

 words, the whole effect of 

 bending, or, as it is scientific- 

 ally termed, of refracting the rays, depends upon the angle 

 of inclination in respect to the surface of the glass at which 

 the ray enters and leaves it. Fresnel's idea was, therefore, 

 to dispense, as far as possible, with the interior substance 

 of the glass, by dividing the lens into portions, and making 

 the several portions thin, while he still preserved in all the 

 same inclinations of the surfaces in relation to the entering 

 and departing ray. 



You will see how this object is effected by the engrav- 

 ing, which shows pretty clearly the nature of Fresnel's 

 contrivance, and the manner in which it operates to pre- 

 serve all the refracting power of a convex lens by retain- 

 ing the several portions of the surface in the right position 

 in respect to the entering and departing ray, while yet the 

 thickness of the glass is kept within reasonable limits. 



