ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 367 



especially by the solution-medium, temperature, movement, time, and 

 quantity of oil. Light- influence was only observed in the case of silver. 



6. The best results were obtained in the ultramicroscopical tests of 

 oils after reaction with gold chloride. In the case of amicroscopic 

 particles there was a reddening ; a blue colouring resulted with sub- 

 microscopic and microscopic particles, combining into a blue-black 

 precipitate ; a yellow coloration followed with other submicroscopic and 

 microscopic particles, which combined into a shimmering and gleaming 

 golden mass. One form only, or two, or all three together can be 

 met with. 



7. From silver solutions either amicroscopic particles precipitated, 

 causing a blue colour of the cone, or submicroscopic and microscpic 

 particles. With the silver reduction, moreover, the reaction with the 

 sulphur contained in many oils gives cone-coloration by reason of the 

 amicroscopic particles. 



8. In reactions with ruthenium chloride the result is either a yellow 

 or a red cone-colouring with the amicroscopic particles, or the sub- 

 microscopic and microscopic particles segregate themselves. 



9. Tests with osmic acid and with platinum chloride gave no practi- 

 cally useful results, and other more favourable conditions for reaction 

 and test are to be sought. 



10. The desired connection of the ultramicroscopical condition with 

 a property expressible in figures or a quantitative reaction was not found, 

 and the question, how fine is the emulsion of which an oil is capable 

 with a given reagent under existing conditions, still remains open. 



A Simple Method for the Determination of the Refraction-Index 

 of Liquids.* — A. Pauly shows how this quantity can be found without 

 costly apparatus and from a small quantity of fluid. The equation on 

 which the calculation is based is : 



w£ 



n:= x /esm 2 Y + iv 2 co& i Y 



where to and £ are respectively the major and minor semi-axes of the 

 ellipsoid of rotation, n the refractive index, and V the angle between 

 the radius vector and the major axis. In calcite the principal refractive 

 indices (with sodium light, iv nn ) are w m — 1*6585, £ Ba =r4864 ; all 

 indices between these values lie on the ellipse. If then a section be 

 taken parallel to the principal axis, all intermediate indices can be calcu- 

 lated if the angle of inclination of the ray be determined. A drop of 

 the fluid under investigation is placed on the calcspar plate and under a 

 coverglass. The polariser is then applied to determine the angle of 

 rotation. The ray through the Nicol now passes in a known direction, 

 and only the refractive index parallel to this direction is effective, and, 

 as the fluid is isotropic, it is the same in all directious. The object-stage 

 with this preparation is rotated until the inequalities of the upper plane 

 or the limits of the periphery disappear. Let A be the angle read off 

 on the circumference of the object-stage at the disappearance of the 

 inequalities, and let B be the reading when the inequalities again dis- 



* Zeitschr. wiss. Mikrosk., xxii. (1905) pp. 344-8 (1 fig.). 



