ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 409 



ordinary as opposed to polarized light. The various methods that have 

 been proposed from time to time for the determination of refractive 

 indices, commencing with that of the Due de Chaulnes (1767), and 

 ending with the more modern methods of Schroeder van der Kolk, 

 Pauly, De Souza-Brandao, Clerici, and Becke, are given in brief, as are 

 also the various modifications of the method of Delesse for the de- 

 termination of the volume percentage of the constituents of rocks by 

 the measurements of areas and lines. No less than eight chapters, of 

 more than one hundred pages — illustrated by remarkably good reproduc- 

 tions of plates from Dr. Hauswaldt's magnificent atlas entitled " Inter- 

 ferenzerscheinungen im polarisirten Lichte " — are devoted to the con- 

 sideration of the examination of sections in plane, polarized, and 

 convergent light. The work of Becke in determining the positions 

 of the isogres in random sections, by making use of the curves of equal 

 velocities (isotaques) and their orthographic projections (skiodromes), 

 is dealt with. The measurement of the optic axial angle by the most 

 approved modern methods receives satisfactory treatment in two well- 

 illustrated chapters, as does also the determination of specific gravity in 

 succeeding chapters. The final chapters deal with the mechanical 

 separation of rock constituents ; micro-chemical reactions ; the cutting 

 and preparation of thin rock sections, and petrographic collections. 



Enough has been said, we doubt not, to convince English petrographers 

 that Dr. Johannsen's book is one that cannot be dispensed with for their 

 future work. 



B. Technique.* 

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



Trap for Free-swimming Organisms.t— Simply stated, this is an 

 arrangement which cuts off the retreat of the creatures after they have 

 been attracted into a small receptacle by light. The first trap used by 

 the writer, S. C. Akehurst, was made of glass, in two pieces. The top 

 is funnel-shaped, and holds about 5 oz. of water. This is attached to a 

 horizontally-placed cylinder, 1 in. diam. and If in. long, the whole 

 being mounted on a stem and foot. Into the cylinder is fitted a glass 

 spigot, which has been ground in to avoid water passing. There is a 

 hole at the bottom of the funnel flask which allows free access of the 

 water to a small well in the glass spigot. When the trap is working, 

 this well opens immediately under the hole at the bottom of the flask, 

 and into this the organisms can enter freely. When desiring to fix the 

 catch, give the spigot a slight turn — the mouth of the well then presses 

 against the side of the cylinder and the contents become locked in. 



To set the trap, fill the flask with pond-water, cover the entire 



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

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

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

 {6) Miscellaneous. 



t Journ. Quekett Micr. Club, xii. (1914) pp. 279-80 (1 fig.). 



. Aug. 19th, 1914 2 K 



