THE PARABOLIC ILLUMINATOR. 



173 



seen in the tracheal system of insects. If any of the 

 transparent larvae of the various kinds of gnat found 

 about ponds in spring-time, be mounted in the elastic 

 gelatine and glycerine jelly (which must be warmed 

 only enough to run, and not kill the insect at the 

 time), on about the third day afterwards all the water 

 is absorbed from the tubes, and they become filled with 

 air. Illuminated by the parabolic condenser, and 

 viewed with the binocular microscope, and a low 

 power, the gnat-larva is a superb object. The body of 

 the insect is but faintly visible, but, in its place, is dis- 

 played a marvellous tracheal skeleton, with each tube 



FIG. 106. A sectional vieio of Wenham's Parabolic Illuminator. 



standing out in perspective, shining brilliantly, like 

 a structure of burnished silver. Unfortunately, such 

 objects are not permanent, for when the whole of the 

 free w r ater dries up, the tracheal tubes either collapse 

 or become refilled with fluid. 



As the blackness of field, and luminosity of the 

 object, depends upon the excess of light from the 

 paraboloid received beyond the angle of aperture of 

 the object-glass, it is found in practice that more and 

 more of the inner annulus of rays from the paraboloid 

 has to be stopped off, until, at last, with high-angled 

 objectives, it is scarcely possible to obtain a black field. 



