THE MICROSCOPE. 



15 



convenient as a hand-lens, but its definition is of 

 course not so good as that of a well-made 

 doublet or achromatic lens. 



Another very useful form of doublet was pro- 

 posed by Sir John Herschel, chiefly like the 

 Coddington lens, for the sake of a wide field, and 

 chiefly to be used in the hand. It is shown in 

 Fig. 9; it consists of a double convex or crossed 

 lens, having the radii of curvature as 1 to 6, and 

 of a plane concave lens whose focal length is to 

 that of the convex lens as 13 to 5. 



Various, indeed innumerable, other forms and 

 combinations of lenses have been projected, 

 some displaying much ingenuity, but few of any 

 practical use. In the Catadioptric lenses the 

 light emerges at right angles from its entering 

 direction, being reflected from a surface cut at an 

 angle of 45 degrees to the axes of the curved 

 surfaces. A ** 



It was at one time hoped, as the precious F *g- 8> 

 stones are more refractive than glass, and as the 

 increased refractive power is unaccompanied by a correspondent 

 increase in chromatic dispersion, that they would furnish 

 valuable materials for lenses, inasmuch as the refractions would 

 be accomplished by shallower curves, and consequently with 

 diminished spherical aberration. But these hopes were disap- 

 pointed; everything that ingenuity and perseverance could 



accomplish was tried by Mr. 

 Varley and Mr. Pritchard, under 

 the patronage of Dr. Goring. 



~ It appeared, however, that the 



great reflective power, the 

 doubly-refracting property, the 

 color, and the heterogeneous 

 structure of the jewels which 

 were tried, much more than 



counterbalanced the benefits arising from their greater refrac- 

 tive power, and left no doubt of the superiority of skillfully 

 made glass doublets and triplets. The idea is now, in fact, 



Fig. 9. 



