ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



243 



changes of temperature to which they were subjected ou the journey 

 without showing any signs of depreciation. 



Measurement of the Refraction Index of Liquids by the Micro- 

 scope.* — M. L. Decombe's method is based on that of Brewster, but 

 is much more precise. The method requires a glass plate with parallel 

 faces L, and a plano-convex lens resting on the plate (fig. 34). A drop 

 of the liquid to be studied is placed at 0, between the plate and the 

 lens. A is a luminous point. By the help of a Microscope, the positions 

 of the images 0' and A' are determined, being (1) the distanpe from 

 the point of contact of plate and lens ; and (2), the 

 distance from the luminous point A. If A be the dis- 

 placement of the Microscope, and v the index of the 

 liquid, it follows that 



a B 



M i 



'A' 



4A 



i 



■ 



Fig. 34. 



where A and B are two positive constants which, in 

 the special case when A is at infinity, have for their 

 respective values A = N, B = R ; N and R expressing 

 respectively the index and the curvature-radius of the 

 lens. The coefficient A = N must be previously de- 

 termined by goniometric methods ; then B can be cal- 

 culated, if v for a known liquid be taken. 



The point 0', being independent of the imperfec- 

 tions of the plate L, of the aberrations of the curved 

 surface and of the nature and opacity of the liquid, 

 can be determined with great accuracy, and can be 

 ascertained, once for all, at the outset of each series of 

 observations. Precision depends particularly on B ; but 

 the author's experiments show that in monochromatic 

 light the error can be easily rendered less than 0"001. 

 To get the second point as accurately as possible, a 

 liquid biconcave meniscus should be employed — i.e. the liquid should be 

 interposed between two convex glasses in contact at their summits, the 

 radii of curvature being chosen in such a manner as to reduce to a 

 minimum the mean spherical aberrations. 



In the author's experiments a cross lightly traced with a diamond 

 on the plate L served as a net for the first point. Various precautions 

 had to be taken, and these are described in the treatise. A Monpillard 

 screen giving green light sensibly monochromatic, was used. When the 

 adjustments have been made, and the constants obtained, it will be 

 noticed that the method requires only a single drop of the liquid ; and 

 that also the extreme tenuity of the layer is serviceable for translucid 

 fluids ; and that the small volume removes difficulties as to temperature. 



Pleochroic Halos.i — F. P. Mennell draws attention to the special 

 interest attaching to this subject, since Professor Joly's suggestion that 

 they are due to the radio-activity of the inclusions round which they 



* Comptes Rendus, cl. (1910) pp. 389-91 (1 fig.), 

 t Geolog. Mag., vii. (1910) pp, 15-19 (1 pi.). 



