108 THE MICROSCOPE AND ITS REVELATIONS. 



Paraboloid of glass that reflects to its focus the rays which fall upon its- 1 

 internal surface* A diagrammatic section of this instrument, showing 

 the course of the rays through it, is given in Fig. 79, the shaded portion 

 representing the Paraboloid. The parallel rays r r' r", entering its 

 lower surface perpendicularly, pass on until they meet its parabolic sur- 

 face, on which they fall at such an angle so as to be totally reflected by 

 it( 2) and are all directed towards its focus, F. The top of the parabo- 

 loid being ground out into a spherical curve of which F is the centre, the 

 rays in emerging from it undergo no refraction, since each falls perpen- 

 dicularly upon the part of the surface through which it passes. A stop 

 placed at s prevents any of the rays reflected upwards by the mirror from 

 passing to the object, which, being placed at F, is illuminated by the rays 

 reflected into it from all sides of the Paraboloid. Those rays which pass 

 through it diverge again at various angles; and if the least of these, 

 G F H, be greater than the angle of aperture of the Object-glass, none of 

 them can enter it. The stop s, is attached to a stem of wire, which, 

 passes vertically through the Paraboloid and terminates in a knob beneath, 

 as shown in Fig. 78; and by means of this it may be pushed upwards so 

 as to cut off the less divergent rays in their passage towards the object, 

 thus giving a black-ground illumination with Objectives of an angle of 

 aperture much wider than G F H. In using the Paraboloid for delicate 

 objects, the rays which are made to enter it should be parallel, con- 

 sequently the plane Mirror should always be employed; and when, 

 instead of the parallel rays of daylight, we are obliged to use the diverg- 

 ing rays of a lamp, these should, be rendered as parallel as possible, pre- 

 viously to their reflection from the mirror, by the interposition of the 

 'bull's eye' Condenser (Fig. 87) so adjusted as to produce this effect. 

 There are many cases, however, in which the stronger light of the concave 

 Mirror is preferable. When it is desired that the light should fall on the 

 object from one side only, the circular opening at the bottom of the wide 

 tube (Fig. 78) that carries the Paraboloid, may be fitted with a diaphragm 

 adapted to cover all but a certain portion of it; and by giving rotation to 

 this diaphragm, rays of great obliquity may be made to fall upon the 

 object from every azimuth in succession. 1 A small glass cone, with the 

 apex downwards, and the base somewhat convex, with a stop in the cen- 

 tre, is fitted by MM. Nachet to their Microscopes for the same purpose;, 

 and performs very effectively. 



106. In order to adapt the Paraboloid for black-ground illumination 

 under Objectives of wide angle of aperture, Mr. Wenham 2 long since 

 constructed a flat-topped paraboloid, fitted to reflect only rays of such 

 extreme obliquity, that they would not pass out of the flat surface of the 

 paraboloid into the under surface of the slide, unless a film of either 

 water or of some liquid of higher refractive index (such as turpentine, 

 or oil of cloves) was interposed between them. When thus enabled to 

 enter the slide, these rays pass on until they meet the cover, from which 

 (in the case of dry-front objectives) they are reflected downwards upon 

 the surface of the object, giving it a bright illumination on a perfectly 

 dark field. The special value of this instrument, however, not being 

 then understood, it was not constructed for sale. The same prin- 



1 By the use of such a diaphragm, or of a large stop with an eccentric per- 

 foration, Mr. G. Williams has succeeded in resolving the transverse striae of 

 Amphipleura pellucida with water-immersion Objectives. See " Journ. of JRoy^ 

 Microsc. Soc.," Vol. iii. (1880), p. 524. 



2 " Transact, of Microsc. Soc.," N. S., Vol. iv. (1856), p. 59. 



