Oblique Illiimiiiatioii. ij 



cerin. The light now passes up through glycerin and slide, 

 and can be focussed on the upper surface. 



Any object placed upon the slide is well lighted, and with 

 rays of great obliquity only. Obviously this paraboloid gives 

 us a perfect dark ground effect, the rays being so very ob- 

 lique that no immersion objective can take them up, and with 

 a dry one the only rays that pass to the objective must be 

 radiated from the object itself. 



It certainly appears, then, that this new " immersion para- 

 boloid " possesses certain points of superiority over any other 

 yet devised, and this seems to be borne out by the results of 

 experience. The fact should not be overlooked, however, that 

 Mr. Wenham clairns to have made a similar instrument as 

 early as the year 1856.* Although Mr. Edmunds denies this 

 claim, it is not easy to understand the distinction he would 

 make between the two instruments, which in principle are 

 surely identical. f We would not detract from the merit due 

 Mr. Edmunds for making the new lens of great practical 

 value at the present day. 



My purpose in describing these instruments is to open a 

 question which seems pertinent at this time, and which has 

 been too much ignored by microscopists as a rule : what kind 

 of illumination is the best — that from below the stage or that 

 from above ? 



Perhaps it is too early to settle the question now, but it 

 certainly is time to consider it carefully. 



First, let us ask, are these, and the many other sub-stage 

 accessories, of sterling value to the observer ? 



There are those who would discard everything beneath 

 the stage but the mirror, and among these are men of no 

 mean authority in such matters. As for them the reason lies 

 in this, that they believe it possible to resolve any test without 

 any other accessory. Well, we must admit that this is true. 

 No test has yet been proposed that cannot be resolved in this 

 way and, by an expert workman, without great difficulty. 

 Nevertheless, while admitting the fact, is it enough to lead one 

 to discard all other devices ? Knowing, as we do, the great 

 difficulties that often arise in making out the exact meaning of 



*Mr. Wenham also placed a hemispherical lens in the cavity of a common paraboloid, and thus 

 made a substitute for the solid lens. 

 fSince this was written Mr. Edmunds has given full credit to Mr. Wenham for this instrument. 



