368 SUMMAKY OF CUKRENT RESEARCHES RELATING TO 



appear after continued rotation. Then V = ^ (A - B) is the angle which 

 the desired refractive index forms with the principal index. The index 

 can also be found graphically when a series of determinations has been 

 made. In this case a curve is drawn whose horizontal co-ordinates give 

 n at intervals from - 01-0 '01, and the vertical co-ordinates give the 

 angle Y at intervals of 10°. Intermediate values would be read off in 

 the usual way. The author gives some tables of results. 



Microscopical Axial-angle Determination of very small Crystals.* 

 E. Sommerfeldt, after describing some of the methods bearing on this 

 subject, recommends as a simple mode the insertion of a scale in a 

 suitable place under the object-stage and parallel to it. This should be 

 used with Lasaulx method, and then both images and scale are well 

 defined. The firm of Fuess has, at the author's suggestion, connected 

 this scale with a condenser, which consists of three plano-convex lenses, 

 the lowest of which carries the scale on its plane-face. The condenser 

 is so calculated that a scale thus placed is sharply defined in the axial 

 image. 



Ultramicroscopical Investigations upon the Colours of Rock 

 Salt.f — H. Siedentopf points out (1) that it is possible to colorise 

 perfectly colourless specimens of rock salt by a process of heating in a 

 vacuum tube in the presence of sodium or potassium vapour (Heraens), 

 and (2) that it is possible by a process of ionisation to affect the colour 

 of a coloured specimen, or impart a tint to a colourless one (Goldstein, 

 Becquerel, Holzknecht, and others). This latter process, however, only 

 seems to produce a surface effect. Siedentopf adopts the method of 

 Heraens by which six or eight rectangular prisms of perfectly clear and 

 clean rock salt, about 10 by 5 by 3 mm., were placed in a completely 

 vacuous combustion tube, and heated for some time to 600° C, so as to 

 render the crystal entirely water-free. By means of a distillation tube 

 consisting of several bulbs united by capillaries, sodium or potassium was 

 distilled over into the presence of the heated rock salt until about a 

 cubic centimetre, with a bright gleaming surface, was adjacent to the 

 crystals. Special care was taken that the surfaces of the crystals should 

 not be soiled with alkali vapour. The evacuation was once more 

 performed, hydrogen at reduced pressure introduced, and the special 

 preparation tube melted off. Fig. 47 shows this preparation tube as 

 supplied by Carl Zeiss.J Its use will be readily understood from the 

 foregoing. The heating is most conveniently done in one of Heraens' 

 vertical electric furnaces, which should be maintained at a constant 

 temperature of about 50-80° below the boiling point of the alkali-metal. 

 This would be about 680° C. in the case of sodium, and about 590° C. 

 for potassium. This temperature is an optimum, for, at a lower tem- 

 perature, too little alkali vapour would be produced, and, at a higher 



* Zeitschr. wiss. Mikrosk., xxii. (1905) pp. 356-62 (4 figs.). 



f Ultramikroskopisch Untersuchungen iiber Steinsalzfarbungen. Verhand. der 

 Deutschen Physikal. Gesell., vii., No. 1421 (1905) ; also as a separate tract under 

 above title. Braunschweig, Vieweg & Sohn. 



% Special circular, Praparatenrohre zur Herstellung optischer Resonatoren aus 

 metallischen Natrium im Steinsalzkristallen. 



