ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 185 



instead of focusing sharply in the image-plane they are deflected (as 

 indicated with the two rays with arrows in fig. 19), and are thus able to 

 enter the low-power objective (E.F. 16 mm.), and finally to reach the 

 eye of the observer. If a mineral grain immersed in a liquid of slightly 

 different refractive index be examined under these conditions of illu- 

 mination, its edges appear in part brighter, and in part darker than 

 tin field. The intensity of illumination of the field is so weak that the 

 illumination of the edges is clearly marked even for differences in re- 

 fractive index of only ± 0*001, and the eye suffers no appreciable 

 strain in making the observation. If now the upper blade be moved 

 away from the edge of the image, a small amount of direct light from 

 the condenser enters the field, and the phenomena produced by oblicpie 

 illumination from the lower stop are observed under reduced field 

 illumination. As the upper blade recedes, the field illumination in- 

 creases, until finally the conditions of ordinary oblique illumination are 

 reached. The phenomena observed under the first set of conditions 

 are. moreover, the reverse of those produced on withdrawing the upper 

 stop ; the edges which appeared bright in the first case are dark in the 

 second, and vice versa. This reversal, caused by the shift of the upper 

 stop, is an additional factor which adds to the sensitiveness of the 

 method. The movable upper stop not only increases the distinctness 

 of the ordinary phenomena of oblique illumination by reducing the field 

 illumination, but it also enables the observer to reverse the phenomena, 

 and to study the slight differences in illumination against a dark field, 

 for which the eye is more sensitive. 



(6) Miscellaneous. 



Merlin, A. A. C. E.— On the Minimum Visible. 



[The author reviews many examples of measurements of very minute 

 magnitudes.] 



Joum. Quekett Micr. Club, xii. (Nov. 1914) pp. 385-92. 



B. Technique.* 

 (1) Collecting- Objects, including' Culture Processes. 



Studies in Amrebic Dysentery.f— In the course of an abstract by 

 Schill (Dresden) of " Studien iiber Amobendysenterie," by K. Ujihara,f 

 the following method is given for the examination, by means of enrich- 

 ment, of encysted amoebae. 



The faecal matter, containing cysts, is first filtered through gauze, 

 and 60 c.cm. of the filtrate is then mixed with 30 c.cm. of glycerin, the 

 specific gravity of the resulting mixture amounting to about 1070. The 



* This subdivision contains (1) Collecting Objects, including Culture Pro- 

 cesses ; (2) Preparing Objects ; (3) Cutting, including Imbedding and Microtomes ; 

 (4) Staining and Injecting ; (5) Mounting, including slides, preservative fluids, etc. ; 

 (<j) Miscellaneous. 



t Centralbl. Bakt. Ref., lxii. (1914) pp. 316-18. 



X Zeitschr. f. Hyg. u. Infectionskrankh. Bd. 77, 1914, p. 329 et. seq. ; and 

 Mitt. d. Med. Gesellsch. zu Tokio, Bd. 28, 1914. 



April 21st, 1915 o 



