ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 515 



In the exact measurement of refractive indices of minute crystal particles 

 by the immersion method it is essential :— 1. That correctly-oriented 

 sections be selected which are normal to at least one principal optic 

 section. 2. That in case oblique illumination be used, only those pencils 

 of light be employed whose direction of propagation is included in the 

 plane normal to the principal optical section ; in other words, the metal 

 sliding-stop should be so inserted that its front edge is parallel to a 

 principal section. 3. That with central illumination special weight be 

 given to the effects along those edges of the grain which trend approxi- 

 mately parallel to the principal section. If these conditions be dis- 

 regarded, the value obtained for the maximum refractive index will be 

 too low, the value for the minimum refractive index too high, while that 

 of any intermediate refractive index, as /?, may be too high or too low. 

 Failure to recognize these factors may lead, especially in the case of 

 strongly birefracting crystal particles, to refractive index determinations 

 which are appreciably in error. 



B. Technique.* 

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



Culture Media for the Isolation of Cholera. f — 0. Lentz has found 

 that dried Dieudonne's blood agar gives bad results in the isolation of 

 cholera vibrios, owing to the binding of the free alkali by the serum 

 proteins of the medium. In the following method, which gives ex- 

 cellent results, the blood-alkali mixture is dried apart from the agar. 

 Fresh defibrinated ox-blood is mixed with an equal quantity of normal 

 potash, and then steamed for half an hour. The liquid is then eva- 

 porated in shallow trays under an air current at 37° C, the residue 

 being ground to powder and kept in a stoppered bottle. Three grains 

 of powder are dissolved in 30 c.cm. water and are added to 70 c.cm. 

 neutral agar. 



H. Yiolle $ uses the following medium for the isolation of cholera 

 vibrios in preference to Dieudonne's blood agar : — Nutrient agar 

 87 parts, glycerin 10 parts, normal soda 3 parts. The medium is easily 

 prepared, keeps well, and is transparent, enabling the observer to dis- 

 tinguish the cholera colonies as small transparent, somewhat rounded, 

 pin-head growths. The vibrios are somewhat short and plump and 

 less motile than normally ; nevertheless, they are easily recognizable and 

 retain their staining properties. Two p.c. pepton-water can be sub- 

 stituted for the nutrient agar, and a good liquid medium thus obtained. 



* 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. ; 

 (G) Miscellaneous. 



t Dent. Med. Woch., xli. (1915) pp. 425-6. 



j Bull. Soc. Path. Exot., viii. (1915) pp. 52-4. 



