292 SUMMARY OF CURRENT RESEARCHES RELATING TO 



If tho instrument is in perfect adjustment, tho readings arc expressed by 

 the formula n = N sin 0, where n is the index required, N is the refractive 

 index of the hemispherical lens, is the angle between the position of 

 the lens for total reflection, and that in which its base is perpendicular to 

 the axis. The apparatus must be graduated by experiment, and the 

 valuo of N is found by observing some substance whose index is 

 known. If N' and 0' are the observed values, and v f their errors, tho 

 above equation becomes n = (N' + v) sin (0' -\-f), or expanding and 



neglecting terms of tho second order r , — N' tan 0' = v tan 0' -}-/N'. 



D ° cos 



From this formula N and / are determined by observation of a number 



of known substances. 



In the refractometer examined by M. Mallard / = 1° 44', but when 

 the correction was applied, indices of refraction were given correctly to 

 two or three units in the fourth place of decimals. 



Apparatus for Microphysical Investigations.* — The following 

 notes are by Dr. O. Lehmann : — 



Warming and preserving the objects.— Fino wire gauze should be used 

 with the author's crystallization Microscope to prevent the cracking of 

 the slide during the heating of the object. To preserve the object the 

 author runs a drop of paraffin round the edge of the watch-glass which 

 covers it, by which it is then hermetically inclosed. For sudden cooling 

 it is advisable to use mercury, in which the slide is immersed with tho 

 cover-glass downwards. 



Change of solubility by pressure. — A Cailletet pump, filled with 

 glycerin, is connected by means of a long copper capillary tube with a 

 glass capillary, which is cemented on the stage of the Microscope with 

 shellac. The glass tube is previously filled with a hot saturated solution 

 and the end of the tube is then sealed. The capillary is placed in a 

 drop of oil on the stage, and covered with a flat watch-glass. After 

 waiting until the conditions are constant, the pressure is suddenly raised 

 to 300 atmospheres, and so maintained ; any crystal in the capillary is 

 then seen to be slowly increasing in size ; after a few minutes this growth 

 ceases, and if the pressure is then withdrawn re-solution will commence, 

 the edges and corners becoming rouuded, and the faces corroded. 



Microscopic determination of capillary pressure. — A fine capillary tube 

 of a diameter less than ■ 001 mm. is connected with an apparatus for 

 regulating the pressure. The apparatus consists of two receivers, one 

 filled with compressed and the other with rarefied air, both connected 

 with the capillary by means of cocks. The pressure is measured by a 

 large mercurial manometer. The end of the capillary tube having been 

 brought into a drop of water upon a slide, and covered with a cover- 

 glass, the pressure is regulated so that the water which has entered tho 

 tube is just driven back to the aperture. The author has used pressures 

 of nearly five atmospheres. 



It may be also proved by this apparatus that the capillary attraction 

 and viscosity diminish as the temperature increases. The diameters of 

 very fine tubes are determined by immersing them in a liquid having the 

 same index as tho glass of which they are made, and measuring with an 

 eye-piece micrometer. 



* Zeitschr. f. Kryst., xii. (1887) pp. 377-410. Cf. Zeitschr. f. Wiss. Mikr., iv. 

 (1887) pp. 115-23. 



