446 Transactions of the Society. 



The specimens exhibited this evening have been produced in this- 

 instrument. Note the slow speed of oscillation — not more than about 

 seven excursions a minute. 



6. Illumination of the stage. — In this experiment a shutter is 

 mounted with two apertures, one of which allows the whole stage to be 

 flooded with light ; the other cuts off the light except from the centre 

 of the stage which is actually under observation. Note the strengthen- 

 ing of the image which results from substituting the small aperture for 

 .he large one. See above, p. 425. 



EXPLANATION OF PLATE YI. 



Fig. 98 (see text, p. 415). 



Figs. I., II., and III. The Bacillus of Anthrax, magnified 400 x2J= 1000 times 



Fig. I. The specimen photographed through an oscillating screen. 



Fig. II. The image seen upon the screen at rest (showing grain of the screen). 



Fig. III. The same object seen in the same instrument without the screen. 



The apparatus used in the production of these photographs was a compounding 

 draw-tube as described (p. 421), fitted with T \ oil -immersion by Beck as the prin- 

 cipal objective, with a J-in. by the same makers as the second objective. These 

 objectives were not selected on any principle of special adaptation to co-operative 

 working, and in fact it will be seen how little adapted they are to work together 

 from the flare spot in Fig. III. 



Fig. III. shows the "peculiarly spotty appearance of the field under excessive 

 magnification," to which Helmholtz refers (see p. 401). With an oscillating screen 

 this apparatus gives a very pleasing image to the eye with amplifications amounting 

 to as much as 8000 diameters. 



