PARABOLIC ILLUMINATOR. 



115 



Fig. 03. 



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 

 Spot-Lens have an angle of 60°, its rays will enter a l-4th 

 Objective of 70°, and the field will not be darkened. 



85. — A greater degree of obliquity may be obtained by the Para- 

 bolic Illuminator* (Fig. 63) now in general use ; which consists 

 of a Paraboloid of Glass that reflects to its focus the rays which fall 

 upon its internal surface. A diagrammatic section of this instru- 

 ment, showing the course of the rays 

 through it, is given in Fig. 64, the shaded 

 portion representing the Paraboloid. 

 The parallel rays r r' r", entering its 

 lower surface perpendicularly, pass on 

 until they meet its parabolic surface, on 

 which they fall at such an angle as to be 

 totally reflected by it (§ 2), and are all 

 directed towards its focus f. The top 

 of the Paraboloid 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 perpendicu- 

 larly 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, so that the object is 

 seen only by the light issuing from itself, and is shown brightly 

 illuminated upon a black ground. The stop s is attached to a stem 

 of wire, which passes vertically through the Paraboloid and ter- 

 minates in a knob beneath, as shown in Fig. 63 ; 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, by which means a black- 

 ground illumination may still be obtained 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, consequently the plowe Mirror should always 



Parabolic Illuminator. 



* A Parabolic Illuminator was first devised by Mr. Wenham, who, 

 however, employed a Silver speculum for the purpose. About the same 

 time Mr. Shadbolt devised an Annular Condenser of Glass for the same 

 purpose (See "Transact, of Micros. Soc." Ser. 1, Vol. iii. pp. 85,132). 

 Both principles are combined in the Glass Paraboloid. 



I 2 



