78 THE MICROSCOPE. 



object obtained, disregarding the precise quality of the 

 light. Then proceed for perfect adjustment, as directed in 

 using Gillett's condenser. 



The Parabolic Reflector. F. H. Wenham, Esq., (Micros. 

 Trans. 1851) proposed a new illuminator, for the purpose 

 of obtaining perfect definition under high powers. Those 

 who have experimented on the subject, may have observed 

 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 ingenious metallic reflector, by which the condensa- 

 tion of lateral light may be effected. 1 



Fig. 44. Wenham' s Parabolic Reflector. 



" The apparatus is shown in section in fig. 44 : a a is 

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



(1) In Vol. IV. 1856, Microscopical Society's Transactions, p. 55, will be 

 found another very instructive and scientific paper, " On the Method of Illumi- 

 nating Opaque Objects under the Highest Powers of the Microscope," by Mr. 

 Wenham. The principle of operation consists in causing rays of light to pass 

 through the under side of the glass slip upon which the object is mounted, at 

 the proper angle for causing total -internal reflection from the upper surface of 

 the thin cover, which is thus made to act the part of a speculum, for throwing 

 the light down upon the under-lying objects, immersed in the balsam on fluid. 



