ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 853 



in this is usually sufficient time to give the bacteria a deep violet 

 colour. Those who have not resorted to this method will be astonished 

 at the facility with which it is practised, and with the variety of forms 

 which may be demonstrated at a moment's notice, without a resort to 

 culture-experiments, or to a search in ditcbes or sewers. The mouth, 

 the rectum, the extremity of the uretha in the male, and the vagina 

 in the female, are constantly supplied with an incredible number of 

 these minute vegetable organisms, and a great variety of forms may 

 be observed, especially in the discharges from the bowels. The 

 slightest possible smear of saliva scraped from the surface of the 

 tongue, of vaginal mucus, or of fecal matter dried upon a slide, stained 

 with violet ink and washed with a gentle stream of water, will furnish 

 ample material for study, and will serve as a practical demonstration 

 of the extensive distribution of the bacteria. 



Bacteria may often escape observation, not only because of their 

 minute size, but because they may have very nearly the same refrac- 

 tive index as the fluid which contains them. Assertions therefore, 

 as to their not being found in certain secretions, &c., will have but 

 little value, unless it is shown that this or some other efficient method 

 of staining has been resorted to, and the objective employed is men- 

 tioned. 



Gunther's Photographs of Pleurosigma angulatum. — Dr. Car- 

 penter has engraved portions of these photographs (x 2000 and 

 central illumination) presented to the Society by Mr. 0. Brandt,* and 

 we subjoin the four figures which he gives. 



Fig. 202 A shows normal hexagonal areolation, areolae bright 

 circles, surrounded by dark hexagons (the "beaded" aspect). 



B. In upper part, areolae and their dark borders graduating from 

 circular to elliptical ; in lower part, dark borders coalescing laterally, 

 so as to give the appearance of continuous vertical lineation. 



C Areolae larger, brighter, and more elliptical, their dark 

 borders coalescing laterally, so as to form very decided vertical 

 lineation. 



D. Transition from hexagonal to triangular areolation, with 

 three series of dark lines, one horizontal and two oblique. 



After discussing the nature of the markings of diatoms, it is added,f 

 "Notwithstanding these considerations, however, it must be freely 

 admitted that there is still considerable uncertainty respecting the real 

 structure of the diatom valve. For it cannot be positively asserted 

 that the focal adjustment which gives the image represented in Fig. 202 



A, is more correct than that which gives the equally distinct images, 



B, C, and D, of other parts of the same valve, of which the last 

 departs in the most marked manner from what is commonly regarded 

 as the normal type. And now that it has been shown that these 

 images are not formed dioptrically, but are resultants of the combina- 

 tion of numerous diffraction-spectra, it is impossible to entertain the 



* See this Journal, iii. (1880) p. 1085. 



t ' The Microscope,' &c., 6th ed. (1881), pp. 333-4. 



