PARABOLIC AND ANNULAR CONDEJjSEES. 



139 



foregoing plans, however, will answer well for objectives of high 

 power, having such large angles of aperture that the light must 

 fall very obliquely to pass beyond them altogether. Thus if the 

 pencil formed by the " spotted lens" have an angle of 60°, its rays 

 will enter a l-4th-inch objective of 70°, and the field will not be 

 darkened. For obtaining a greater degree of obliquity, Mr. 

 Wenham has contrived a Parabolic Speculum,^ having its apex 

 cut off, so that the object might be placed in the focus, to which 

 all rays parallel to its axis are reflected ; and the direct rays being 

 checked by a stop placed behind it, the object is illuminated only 

 by those which are reflected to it from all sides of the interior 

 of the parabola at a very oblique angle. As the thickness of the 

 glass slide on which the object is mounted, was found by Mr. 

 W. to produce a very sensible aberration in the rays converging 

 towards it, he interposed a meniscus lens, having such a curva- 

 ture as to produce a counteracting aberration of an opposite kind. 

 The circular opening at the bottom of the wide tube (Fig. 40) 

 that carries the speculum, may be fitted with a diaphragm, 

 adapted to cover any portion of it that may be desired ; and by 

 giving rotation to this diaphragm, rays of great obliquity may 

 be made to fall upon the object from every azimuth in succes- 

 sion (§ 60). A like purpose was aimed at in the Annular Con- 

 denser of Mr. Shadbolt,^ which consists of a ring of glass, whose 

 surface was so shaped as to present a prismatic section ; the in- 

 clination of the outer side being such as to produce a total re- 

 flection 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 combina^ 

 tion of both methods is adopted in the Parabolic Illuminator 

 (Fig. 40), now supplied by Messrs. Smith and Beck ; for this 

 consists of a paraboloid of glass, resembling 

 a cast of the interior of Mr. Wenham's para- 

 bolic speculum, but reflecting the rays which 

 fall upon the outer surface of the glass, like 

 Mr. Shadbolt's annular prism. It has the ad- 

 vantage of being more easily constructed than 

 the parabolic speculum, and is little, if at all, 

 inferior to it in performance ; but it requires 

 that an appropriate " stop" should be adapted 

 to it, for each objective with which it is to be 

 used : whilst in Mr. Wenham's speculum, the 

 requisite adaptation for the angular aperture 

 of the objective is made by altering the posi- 

 tion of the stop by means of the central stem ; 

 the effect of which alteration is to cut off a 

 larger and larger proportion of the least ob- PaiaboUo lunminator. 

 lique rays, the more nearly the stop is approximated to the ob- 



Fis. 40. 



' "Transactions of the Microscopical Society" (1st Series), 

 ^Op. cit. p. 132. 



vol. iii, p. 85. 



