515 



the object, giving opaque illumination," and secondly that " the 

 spherical aberration is inconsiderable." 



Obviously the object must be placed slightly within the focus 

 of the condenser, and an annular ring of light projected upon 

 the inside of the cover-glass, there to be totally reflected back 

 again to a focal point on the top of the object. 



As no provision is made by Mr. Stephenson to correct the 

 spherical aberration which arises at the concave surface of 

 the mirror, the amount can hardly be inconsiderable ; on the 

 contrary, it is large enough to cause the marginal rays to focus 

 underneath the slip. 



It occurred to me that, working somewhat in the manner 

 I have described in a previous paper, the spherical aberration 

 might be reduced and brought within reasonable limits. Thiii 

 has been done, and the illuminator that Messrs. Baker will 

 exhibit to-night will be seen to be particularly free from 

 aberration. 



This condenser focuses parallel rays directly upon the object ; 

 its N.A. is 1*3 and its focus 8 mm. A glance at the figure 



will show that a convex reflecting surface which is also a. 

 concave refracting surface has been introduced to neutralise 

 the aberration of the concave mirror. 



The optics of the device are rather interesting, for it appears 

 that no spherical curve will give absolute aplanatism. What the 

 precise curve necessary to do this is I am unable to tell you^ 



