ILLUMINATORS. 203 



objects must be under a certain size, otherwise no %ht will 

 pass around them to reach the Lieberkuhn; but, with the 

 former, it matters not how large they may be, as the light is 

 thrown down obliquely upon them, in the same manner as if 

 the condensing lens were employed. As a general rule it 

 may be stated, that the side-reflector is most useful with 

 powers up to the half-inch, the Lieberkuhn with the higher. 



It must be borne in mind that those objects in which rich 

 tmts of colour prevail, or others whose surfaces in any way 

 decompose the light, must be mounted in such a manner, that 

 they may be turned in every possible direction, in order that 

 the rays of light may fall on them at very oblique angles ; a 

 disc of card or cork, with a pin through it, will be the best 

 plan to adopt, as then they may be inclined at any angle, 

 either by turning the pin or shifting the forceps in which the 

 pin is held. The mounting on flat slips of glass, as shown in 

 figs. 124-5-6, cannot be advantageously employed with such 

 objects. 



Condensers or Illuminators of Recent Construction. — The 

 two following Condensers or Illuminators, like that of Mr. 

 Wenham, before described in page 116, are for the purpose 

 of illuminating objects by oblique light, equal on all sides, so 

 that no shadow is produced ; whilst in the prism of Nachet, 

 described in page 120, one side only is illuminated. This 

 latter plan, however, had been already to a certain extent 

 accomplished in two ways, by turning the mirror very far to 

 one side, and by the background illtimination of the Kev. 

 J. B. Keade, described in page 1 94. 



Noberfs Illuminator. — This, as shown by fig. 132, consists 

 of a thick plano-convex lens, A B, in the centre of the convex 

 surface of which a deep concavity is made: when in use, 

 the cell in which the lens is contained is fitted to the 

 framework of the achromatic condenser, the plane surface 

 being turned towards the object. All the rays, b b, which fall 

 upon the concave face are dispersed in the manner shown 

 by V V, whilst those, a a, a a, falling on the convex surface 

 are brought into a focus at C. By this arrangement a 

 hollow cone of light is obtained, the concave surface being 



