AMICI'S PRISM, ETC. xxi 



Its use is to condense the light upon opaque objects. When used, it is placed between the 

 object hung upon the slide under the microscope and the lamp or other source of light, 

 which should be about 6 or 7 inches from the object, the plane surface of the lens being at 

 right angles to the direction of the rays of light, and next the object ; and the lens must be 

 brought so close to the object that the focus falls upon the latter. Sometimes a "small 

 condensing lens " is used to concentrate the lisrht already transmitted through the lar<?e 

 condenser : this is usually fixed to some part of the microscope. A doubly convex lens of 

 much longer focus than the bull's-eye lens, about 7 or 8 inches, will be found very useful 

 for condensing the light upon the mirror when the achromatic condenser, stops, &c. are 

 used with the highest powers. The arm of the bull's-eye lens maybe adapted to hold 

 either or both of the lenses. 



Amici's prism is sometimes useful for throwing very oblique light through a transparent 

 object. It consists of a flattened-triangular glass prism, the two narrower sides of which 

 are convex. The third and broadest side forms the reflecting surface. The prism may be 

 attached to a separate stand, or to the secondary stage. It is sometimes mounted on a 

 pillar placed beneath a large brass slide, perforated in the centre. A triangular prism 

 mounted in either of these ways forms a Reade's prism, and is used in the same manner. 

 Amici's prism exerts a condensing as well as reflecting action. 



Licbcrliiihn. — Some opaque objects may be well illuminated by a lieberkiilm or silver 

 cup ; by which the light, first reflected by the mirror upon the concave surface of the 

 cup, is afterwards reflected upon the object. It is not adapted for higher powers than 

 the \ inch. 



Wen/tarn's Parabolic Reflector, — The discovery of the importance of excluding the 

 central rays of light, and using a central stop for this purpose, is due to F. II. Wenham, 

 who invented an apparatus in which this principle is taken advantage of. It consists 

 of a brass tube fitted beneath the stage in the place of the ordinary achromatic condenser, 

 terminated above by a hollow truncated cone the perpendicular section of which forms a 

 parabola, with an internal polished silver reflecting surface. At the base of the parabola 

 is placed a disk of thin glass, in the centre of which is cemented a dark well. In use, the 

 central rays are stopped b} T the dark well, whilst the lateral rays, passing up the tube, 

 impinge upon the parabolic surface, from which they are reflected upon the lower surface 

 of the object. This apparatus, as modified by Shadbolt, is constructed of a solid cylinder 

 of glass terminating above in a cone the surface of which has the form of a parabola and 

 replaces the silver reflectiug surface — and is the form now generally used. In objects 

 viewed under this or any other form of black-ground illumination, the light reaching the 

 eve is all reflected from certain suitably inclined surfaces of the object. This may be 

 proved by placing a polarizer beneath the reflector, selecting as the object some small 

 strongly polarizing crystals. On applying the analyzer, no colour will be seen, showing 

 that the light has not passed through the object. Hence care must be taken in drawing 

 conclusions from the appearance^. 



Brooke s Reflecting Apparatus. — The purpose of this is to illuminate objects by reflected 

 light, so that they can be examined with the highest power. It consists of two parts ; the 

 first is essentially the same as the apparatus proposed by Wenham. The second consists 

 of a small, flat, circular metallic mirror (a flat lieberkiilm), perforated to admit the lower 

 end of the object-glass, upon which it slides, and so arranged that the reflecting surface is 

 in the same plane as the lower surface of the object-glass. When in use, the light is 



