468 Intelligence and Miscellaneous Articles. 



The above results show that the phosphorographic method per- 

 mits, when the phosphorescent substances are properly chosen, of 

 the investigation of the infra-red part of the spectrum as far as 

 thermoscopic and further than chemical methods. This method 

 has, further, an advantage that the others do not possess, namely 

 that of giving a complete image, at one time, of the part which is 

 to be studied. — Comptes Rendus, September 1, 1884. 



INVESTIGATIONS ON RADIANT HEAT. BY H. SCHNEEBELI. 



In vol. lxxxiv. of PoggendorfPs Annalen, p. 411 et seqq., Svan- 

 berg describes a simple and very sensitive method for investigating 

 radiant heat. 



My object in the following experiments was to test the accuracy 

 and capability of this method, and at the same time to solve experi- 

 mentally some questions in radiant heat by its means. 



Langley and Baur have in recent times used Svanberg's method 

 to decide certain questions which present themselves in radiant 

 heat, and have described arrangements for adapting the method to 

 the various objects. 



The arrangement which I chose in my experiments is the 

 following : — 



A grating of thin tinfoil blackened by platinum chloride and 

 enclosed in a box with movable lid formed the resistance to be 

 irradiated. A Daniell's element was the electromotive force, and 

 the mirror-galvanometer in the bridge was adjusted with such 

 delicacy that a displacement of the contact-key of 1 millim. pro- 

 duced a deflection of some hundred divisions. The resistance 

 in the circuit in which was the battery was kept constant by an 

 intercalated rheochord, and the current-strength was continually 

 controlled. 



Various circumstances led me to investigate the. radiation in the 

 following manner : — the tinfoil grating was exposed to radiation 

 for ten seconds, the current was then closed, and the deflection 

 read off. The slide was of course so arranged beforehand that, on 

 closing and opening before the radiation, the needle was at rest. 



There is an evil in the method, which I will at once mention, 

 that the small current in the bridge never continues for long ; so 

 that even with a careful protection of the sensitive parts of the 

 current there is no guarantee that during the ten seconds in which 

 the radiation continued there is a null current even when the tem- 

 perature of the grating is unaltered. It seemed undesirable to 

 use a shorter time for radiation, since the temperature of the 

 grating is a function of the time, and increases very rapidly with 

 the time, especially in the beginning, and thus slight inaccuracies 

 in the time of exposure would produce great errors. The fol- 

 lowing numbers show this : — 



