Notes. 157 



Swrirella gemma, 0*75; and 1 * 30 with crescent stop for the Amphi- 

 pleura pellucida. 



The Pleurosigma angulatum, Surirella gemma and Amphipleura 

 pellucida were taken from the yellow (realgar ?) mounts of Thum. 

 The Coscinodiscus asteromphalus was mounted in styrax. 



The Barnet extra rapid orthochromatic plates were mostly used. 



Dark Field Illumination. 

 By J. W. Gordon. 



When lecturing at the Royal Institution on February 10 last, 

 I demonstrated the production of dark field illumination in a 

 picture thrown by a projection Microscope, the darkening of the 

 field being, in fact, produced by the introduction of a diffraction 

 fringe. My object was to obtain the improved resolution, which 

 is, on theoretical grounds, to be expected on a background illumi- 

 nated by diffracted light ; but, in fact, I found in rehearsal that 

 I obtained such strongly accentuated representation of refracting 

 surfaces, that I was obliged to confess that the experiment 

 succeeded too well, and the result which I had looked for was 

 masked by an unexpected effect, which at that ime I could not 

 explain. The experiment seemed to me to be none the less in- 

 teresting on that account, and I therefore showed it as a matter of 

 curiosity, believing that I was then demonstrating it in public for 

 the first time. 



A week or two later I lighted by chance upon a reference which 

 sent me to a paper contributed by Professor Toppler to Poggen- 

 dorff s Annalen for the year 1867 (p. 33), and there I found my 

 experiment described, with immaterial variations, and fully and 

 most lucidly explained. 



It then appeared that the diffractor which I had been using 

 was equivalent to a post-objective stop. In the case of the 

 projection lantern the objective angle was considerably wider 

 than the condenser angle, and therefore dark field illumination 

 could not have been obtained — if I had tried to get it — by the 

 usual expedient of a stop in the aperture of the condenser. But 

 for the same reason, a stop shutting out the middle of the objec- 

 tive darkens the field, while allowing strongly refracted rays to 

 pass round its edge in the marginal zone of the objective ; and 

 thus an alternative position was indicated for the dark field stop 

 available for use with objectives of high N.A. 



