96 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the scale, and (2) of the reflecting surface to be tested with the centre 

 of the graduated circle. This latter is accomplished by a small pro- 

 jecting arm with a platinum point. 



Liquids are placed in a little glass capsule on the levelling table, 

 which is adjusted until the platinum point, indicating the centre of the 

 circle, just touches the liquid surface, Bodies having an irregular, 

 granular, or crystalline surface, if fusible, are melted. This is accom- 

 plished by placing them in a small capsule of metal or porcelain, which 

 is heated by a current of steam or an electric current traversing a 

 platinum wire coiled round the capsule. In practice a difficulty occurs 

 in determining the precise angle of maximum polarisation ; for the 

 extinction of the reflected ray seems to spread over a narrow region 

 rather than to occur at a definite point. This error, however, can be 

 lessened by careful attention to the parallelism of the incident rays and 

 homogeneity and intensity of the light. The author employed a small 

 direct vision-prism spectroscope in the collimator and obtained a sharp 

 spectrum, using, of course, a very brilliant source of white light. In this 

 way he hoped to obtain the angle of extinction for a definite colour and 

 thus see a dark band pass across the spectrum, as the polarising angle for 

 such colour was reached. In the case of bodies of very high dispersion, 

 such as nitroso-dimethyl-aniline, the dark band is sharp and well-defined. 

 But in the case of bodies of low-dispersive power, a faint broad shadow 

 is observed to move across the spectrum, the best position to read being 

 when the shadow is in the green or greenish-blue ; results can then be 

 obtained within 20' to 30', even with this preliminary apparatus. 



Pringsheim's Yellow Filters.* — E. Pringsheim, jun., constructs 

 his filters in the following manner : White glass plates (e.g. old photo- 

 graphic plates) are thoroughly cleaned by a solution of potassium 

 bichromate in concentrated sulphuric acid, rinsed in running water, and. 

 the future disk-side downwards, dried by being placed obliquely on 

 blotting-paper. This cleaning facilitates the future adhesion of the 

 gelatin layer. Every speck of dust is to be avoided. A deep reddish- 

 brown solution in distilled water to which 20 p.c. gelatin has been 

 added is filtered in a steam chamber, and a little glycerine at the rate of 

 a single drop per 100 c.cm., is added to prevent undue brittleness to the 

 layer. Some boric acid is also added to prevent growth of moulds. 

 Boric acid is too weak to influence the colour, but must be added 

 sparingly as it is apt to crystallise out in drying. The cleaned glass 

 plates are set out on a larger glass plate accurately levelled. The gelatin 

 solution is poured on to the middle of the plates, and must be hot, so 

 that the application may be uniform in thickness. Any want of success 

 may usually be made good by heating up the unsuccessful part on an 

 asbestos layer. "When the gelatin has solidified the plates are kept 

 obliquely in a dry place as dustless as possible. With rare exceptions 

 the gelatin layer will be found so uniformly applied, that, when held 

 between the eye and a newspaper, the printing seems scarcely affected. 

 Two such plates may be turned inwards and cemented by Canada 

 balsam. 



The author not only uses his filters for the windows of a box for 

 studying the heliotropism of plants, but applies them to the study of 



*^Ber. Deutsch. Bot. Gesell., xxviA. (1908) pp. 556-65 (4 figs.). 



