466 Prof. Draper on the Interference Spectrum^ 



visual organs, — beatwhich disturbs a tbermometer, — a titbonic 

 ray which produces the decomposition of iodide of silver, and 

 a phosphorogenic ray which causes the sulphuret of lime to 

 glow. Even after the spectrum region is pointed out, we re- 

 quire for the distinct understanding of the case a further de- 

 termination of the active principle involved ; whether it has 

 been light or heat, tithonicity or the phosphorogenic prin- 

 ciple. 



As is well known to the readers of this Journal, 1 have en- 

 deavoured for some time past to show that the rays occurring 

 in the sunbeam, and passing now under the names of titbonic 

 and phosphorogenic, are entitled to be regarded as distinct 

 imponderable principles. The main object of the memoirs I 

 have published, has been to bring these from their former ill- 

 defined and indistinct position, and, by giving them specific 

 designations, to have their title to the same rank as light and 

 heat recognised by chemists. The names which I have sug- 

 gested might, perhaps, have been amended ; but after all it is 

 with facts that we deal; and, provided our words are eupho- 

 nious, and are connected with no erroneous theories, they may 

 answer the purpose until the progress of discovery gives us a 

 clearer view of the whole subject, and enables us to employ 

 better. It is very probable that other imponderable princi- 

 ples will be recognised, and, for the present, trivial names are 

 more appropriate for them than theoretical ones. 



In this memoir I propose to confine my observations to the 

 following propositions : — 



I. To describe the photographic and tithonographic phse- 

 nomena of the interference spectrum, and to suggest there- 

 from a suitable subnomenclature for actino-chemistry. 



II. To determine the laws of absorption of the tilhonic rays. 



As the laws of absorption are at the foundation of actino- 

 chemistry, I shall consider this latter proposition under two 

 heads: — 1st, to determine the changes which the ray itself un- 

 dergoes; 2nd, to determine the changes which happen to the 

 ponderable matter or sentient surface. These observations 

 will, for the present, be limited to titbonic, photic, and thermic 

 absorption. Of phosphorogenic absorption I shall speak here- 

 after. 



I. Phmiomena of the Interference Spectrum. — The prismatic 

 spectrum, even when every precaution has been used to obtain 

 it in a state of purity, its fixed lines being visible, is liable to 

 lead us into many errors. As respects its luminous or photic 

 properties, we cannot determine the liistribution or intensity 

 of the light, because the violet extremity is unduly dilated. 



