PROCEEDINGS OF THE SOCIETY. 765 



its general purport, illustrating bis remarks by drawings upon tbe 

 board and by reference to tbe examples sbown under Microscopes in 

 the room, for tbe use of wbicb be was indebted to Messrs. Beck. He 

 also read an extract from a letter received from Dr. G. Johnstone 

 Stoney, in which the writer remarked that the strong reflection from 

 the lens surfaces of the objective, to which attention w^as drawn in 

 the paper, was very conspicuous and troublesome in the use of a 

 vertical ilhirainator, as the reflection in that case was given off by the 

 upper surfaces of the lenses and received directly by the eye of the 

 observer. Dr. Stoney also expressed the opinion that the coloration of 

 Pleurosigma aiigii,l(Uum was capable of another explanation than that put 

 forward in the paper. 



Mr. (rordon, in view of Dr. Stoney's criticism of his reference 

 to P. angtd((tum, showed upon the screen two photographs of P. 

 formosum, one of which was taken with a narrow-angle objective, 

 and the other under one with a high angle ; in the one case it was 

 shown as having black dots, and in the other these appeared white. 

 He mentioned also that when tbis diatom was illuminated by light 

 from a light source of annular form, the source of light being focused in 

 the plane of the stage, its dark centre was visible through the specimen 

 as a patch in the field of the instrument when the condenser beam was 

 cut down to a narrow angle, but that if the angle were opened up so as 

 to yield strong reflection from the objective, the dark patch disappeared, 

 but the bright dots overlying the dark patch still retained minute black 

 centres, evidently due to small foramina in the silex, through which the 

 dark background could be seen even when the upper surface of the silex 

 was so refulgent that the dark patch could not be seen through its 

 substance. He submitted this as being an undeniable instance of the 

 striking effect of the top light from the objective in modifying the 

 appearance of a diatom. 



Mr. Conrady said that the usual explanation of the brown ap- 

 pearance of P. angiilatum under low powers was derived from the 

 fact, easily ascertained by looking down the tube, that this diatom 

 transmitted light of a distinctly brown colour, whilst the light difl'racted 

 by it showed a decided preponderance of blue. Low-power objectives 

 transmitted only the brown direct light, and therefore showed the 

 diatom of a uniform brown colour. As soon as the diffraction-spectra 

 began to enter, their preponderating blue light compensated the brown 

 of the direct light, and produced an approach to whiteness in proportion 

 to the quantity of diffracted light admitted. The explanation of the 

 white dot Ijy light reflected back by the objective, did not appeal to 

 him ; it w^as well known that all these dotted structures had a double 

 focus, known as the white dot and the black dot respectively, and he 

 had indeed noticed that the photograph shown by Mr. Gordon in this 

 connection, whilst showing white dots in the upper half, showed tlie 

 black dot, somewhat out of focus, on the other side of the central ril). 

 As both halves of the diatoms were necessarily under the same con- 

 ditions as to illumination, Mr. Gordon's explanation was obviously 

 wrong. Mr. Gordon's suggestion of using the tiny black dot obtained 

 by double reflection as a test of resolving power, was futile. The 

 seeing of a single object, whether dark or bright, was a question of 



