THE PARABOLIC REFLECTOR. 



179 



The Parabolic Reflector. Mr. F. H. Wenham (Micros. 

 Trans. 1851) proposed a parabolic illuminator, for the pur- 

 pose of obtaining perfect definition under high powers. 

 Those who have experimented on the subject, may have ob- 

 served that there is something in the nature of oblique light 

 reflected from a metallic surface particularly favourable for 

 the purpose of bringing out minute markings, which may, 

 in some measure, be attributed to the circumstance of light 

 so reflected being purely achromatic. In order to render 

 this property available, Mr. Wenham contrived a very in- 

 genious metallic reflector, by which the condensation of 

 lateral light may be effected. 1 



Fig. 104. Wenham' s Parabolic Reflector. 



" The apparatus is shown in section in fig. 104 ; a, a is 

 a parabolic reflector, of a tenth of an inch focus, with a 



(1) Mr. Shadbolt contrived a modification of Wenham's Reflector, and called 

 it a <f Sphero- Annular Condenser;" which consists of a ring of glass whose 

 surface was so shaped as to present a prismatic section, the inclination of the 

 outer side being such as to produce a total reflection of the rays impinging on 

 it, and to direct these through the inner side of the ring, so as to fall at a very 

 oblique angle upon the object from every azimuth of the circle. A combination 

 of both methods is adopted in the parabolic illuminator now very generally 

 used. This consists of a paraboloid of glass resembling a cast of the interior 

 of Weuham's Parabolic Speculum, but reflecting the rays which fall upon the 

 surface of the glass like Shadbolt's Annular Condenser. 

 N 2 



