34 THE MICROSCOPIST. 



mitting a large pencil of light with great distinctness, having 

 an angular aperture of from 35 to 50. Mr. Holland's 

 Triplet, used in the same way, is capable of transmitting a 

 pencil of 65 with distinctness and correctness of definition. 

 The achromatic object-glasses, as first proposed by Mr. Lister, 

 have however superseded all other attempts to improve the 

 compound microscope, and have raised it from the condition of 

 a mere toy to be the most valuable instrument of scientific 

 research. They are made of plano-concave flint, and double- 

 convex crown glass lenses, cemented together. Three com- 

 pound lenses form the object-glass for a microscope, as repre- 

 sented by Fig. 9, a, &, c. In object-glasses of a high power, 



Fig. 9. 



the anterior compound lens, a, has sometimes an adjustment to 

 render it suitable for objects either uncovered or covered with 

 glass of various thickness. The object-glass, thus made, is not 

 quite achromatic, being rather over-corrected as to color, but 

 is finally corrected by using the Huygenian eye-piece, shown in 

 Fig. 10. 



This eye-piece consists of two plano-convex lenses A, B, with 

 their plane sides next the eye. In the focus of A is the dia- 

 phragm or stop, C. The proportions of the focal lengths of 

 these lenses should be as 3 to 1, and their distance apart, one- 

 half the sum of their focal distances. Thus if B be three 



