332 



a measurement of the intensit}^ of the light we have only to compare 

 the density of the image of the TYNDALL-cone with this scale. 



A small NKRNST-lamp I found to work very well as a light- 

 source; by means of some lenses its light was focussed to a well- 

 defined pencil, which was sent through a closed glass cuvette with 

 smooth walls and filled with the solution to be examined. In order 

 to regulate the temperature the cuvette was placed on a thermostat, 

 a cover of asbestos preventing as much as possible the influence of 

 the temperature of the surroundings. The cover was removed only 

 while the pictures were being taken. This ari-angement as well as 

 the photographic apparatus was unaltered during the whole series 

 of exposures, so as to yield perfectly comparable results. 



In order to obviate any deteriorating influence of an occasional 

 inconstancy of the Np:HNST-lamp or of some error in our procedure, 

 an exposure was made together with the TYNDALL-phenomenon on 

 the same plate, of a surface, lighted directly by the lamp. The 

 density of this comparison surface must be the same on the various 

 plates of a series in order to draw conclusions from the differences 

 in the pictures of the TYNDALL-cones '). 



After some preliminary experimentation with an aqueous gelatin- 

 solution, demonstrating that a fall of the temperature causes a 

 considerable rise of the intensity, I invariably used solutions of 

 gelatin in glycerin, because a previous investigation of some other 

 properties, especially the viscosity, had proved how their condition 

 is inrtuenced by the temperature. An advantage of these solutions is 

 that, in closed vessels, they keep very long without any alteration 

 attributable to evapoi-ation or lo micro-organisms. For the solution 

 I used commercial glycerin containing about 30"/^ of water and 

 o-elatin, which had been washed long enough, to remove the salts 

 and had then been dried again. After careful filtration the solution 

 was put in the cuvette. Then it still contained a large number of 

 air-bubbles, which could be removed in vacuo at 70°. 



In all gelatin-solutions that were examined, the incident pencil 

 generated a beautiful cone. In diluted solutions nearly the same 

 intensity is observed over its whole length ; in more concentrated 

 solutions (10 "/J the intensity diminishes rapidly but evenly, as the 



1) For the pictures I used the very sensitive plates of Lumière ("étiquette violette' ). 

 Time exposure 30 sec. Development v^^lth oxalate of iron after Eder for 2 minutes. 

 Fixation in a hyposulphite solution. Development with oxalate of iron is preferable 

 since it yields perfectly black tints, whereas organic developers often act upon the 

 tone of the image. 



