331 



spectrum. It is positive in the green and negative at the two ends. 

 The zero points are near 477 //f< and 640 n}i. The form of tlie curve, 

 also, indicates a minimum in the ultra-violet. (Fig. 5, o points). 



+1',5 

 + 1' - 

 +0',5 



Franklinite 

 rv(t) . / =515 



0' 



' ! L 



-70° -50° 



+50- 



Fig. 6. 



The rotation decreases very slowly from — 78° to 0°. It then 

 falls off rapidly, vanishing between 60° and 65°. (Fig. 6). In agreement 

 with this WoT.oGDiN found 61° for the transition temperature. 



Physiology. — "Oil the Tx'STi ma. -phenomenon In gelatin-solutions." 

 By L. Arisz. (Communicated by Prof. H. Zwaardemakerj. 



(Communicated in the meeting of June 28, 1913). 



In watching the TYNDALL-phenomenon in a 1% gelatin-solution 

 Prof. Zwaardemaker observed a greater intensity of the diffused light 

 after the change from sol to gel than before the gelatification. 



This prompted me to investigate the TYNDALL-phenomenon more 

 closely. 



For a determination of the intensity of the light a method was 

 adopted that has been suggested by Hartmann ^). My mode of pro- 

 cedure was the following : A series of photographs were taken of 

 the TYNDALL-phenomenon .at right angles to the incident pencil and 

 the intensity was calculated every time from the grade of blackness 

 of the picture. This estimation was based on the principle, that 

 when a number of phites, as uniform as possible, are exposed under 

 precisely the same conditions of time, temperature, development and 

 further treatment, the density of the negative will depend only on 

 I he intensity of the light. 



Different portions of a plate being successively exposed at various 

 known distances to the same constant light-source for an equal space 

 of time, afford a suitable scale of various grades of blackness. For 



1) Zs. f. Tnstrum. 19. 97. 18S9. 



