Cata-dioptrie Immersion Illuminator. By J. W. Stephenson. 209 



disposes of three-fourths of the spherical aberration of a concave 

 mirror of these dimensions, and enables one to use light which is 

 practically parallel; and in the second place it has the very 

 obvious one of securing greater aperture, which is the primary 

 consideration. 



As fig. 44 shows, rays which enter the flint glass horizontally 

 are reflected at the silvered surface of the crown-glass segment, and 

 apertures are thus obtained ranging from • 77 to 1 • 644 N.A. in 

 flint, and 1*512 N.A, in crown. 



To obtain the smaller apertures, the plane surface of the under 

 part of the flint is utilized by cementing to it a segment F (rather 

 more than half) of a plano-convex lens of radius • 25 in., with a 

 focus in crown 0*04 in. above the upper surface of the flint, and 

 therefore at the upper surface of a slide having a thickness of 

 0-04 in. 



In order that the rays may be received from the mirror 

 beneath (and not horizontally), a small right-angle reflecting prism 

 G, is placed with one of its sides opposite to and parallel with the 

 receiving side of the flint glass. 



Stops of any form or kind can be used. They may consist 

 simply of a piece of brass with round holes, say 1/9 or 1/10 in. 

 in diameter, sliding in a groove, giving difierent obliquities, or 

 they may be arranged around and gradually approaching the axis 

 of a rotating wheel, or what is perhaps the simplest and best 

 form, one hole in a piece of brass sliding up and down in front of 

 a slot; but in any case, whatever the stop may be, it must be placed 

 between the prism and the flint surface of the lens. 



In the construction of illuminators it is of some importance 

 that the surface by which they are connected with the slide should 

 be as large as conveniently may be, and not of the small dimensions 

 of many illuminators. The aperture is of course controlled by the 

 ratio between the thickness of the slide and the semi-diameter of 

 the lens, when, as may be assumed, the illuminator is centered. 

 The following table shows how little the thickness of the slide 



Thickness of 

 Slide. 



inches. 

 0-03 

 0-04 

 0-05 

 0-06 

 0-07 

 0-08 

 0-09 



Aperture. 



In Flint, 

 (n = 1 • 652) 



1-644 N.A. 



1-637 



1-629 



1-620 



1-609 



1-596 



1-582 



In Crown. 

 (n = l-52) 



512 N.A. 



507 



499 



490 



480 



469 



456 



Ser. 2.— Vol. V. 



