PARABOLIC ILLUMINATOR 



317 



parallel rays r r' r' f (fig. 259), entering its lower surface perpendicu- 

 larly, pass on until they meet its parabolic surface, on which they fall 

 at such an angle as to be totally reflected by it, 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 perpendicularly 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 .-ingles : 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 8 is attached 

 to a stem of wire, which passes vertically through the paraboloid 



FIG. 260. Parabolic 



illuminator. 



FIG. 259. 



and terminates in a knob beneath, as shown in iig. 2ti<) ; and by 

 means of this it may be pushed upwards so as to cut oil' the less 

 divergent rays in their passage towards the object. It is claimed 

 that this instrument has great capabilities of giving dark -ground 

 illumination with lenses of ' wide apertures ; ' but that has application 

 to the lenses contemporary with its introduction, and not to wide 

 apertures as applied to the lenses of to-day. In comparison with 

 what can be done with condensers it suffers greatly after we pass 

 the Tf-inch objective, although it does give excellent results with 

 very low powers such as 1-inch, H-inch, 2-inch, and 3-inch 

 objectives when employed to illuminate large objects such as whole 

 insects, because this instrument gives more diffusion of light over 

 the whole of a large object than a condenser does. 



Polarising Apparatus. In order to examine transparent objects 

 by polarised light, it is necessary to employ some means ' 



