Polytechnic Association Proceedings. 625 



when first emitted are invisible, may be made to display a beautiful 

 blue color. The phenomenon of phosphorescence is explained by 

 supposing that light gives to the molecules of a sensitive body a 

 vibratory motion, which continues for some time after the exciting 

 cause has been removed. Mr. Lea asserts that there is not the 

 slightest reason to doubt that bodies may be endowed with the 

 power of being impressed by the actinic rays, and of retaining them, 

 precisely as bodies may the illuminating rays. Herein lies the 

 explanation of the physical or latent image. It is simply a phos- 

 phorescence of actinic rays. 



Pure iodide of silver {argamav) undergoes no decomposition by 

 light when thoroughlj^ isolated from all substances, organic and 

 inorganic, which are capable of aiding in affecting reduction. But 

 if exposed to light, it continues for a certain time thereafter to 

 retain the vibrations it received; and, just for so long as these 

 vibrations continue, will it be instantly decomposed if brought into 

 contact with any substance which would have caused its decompo- 

 sition had the two been subjected to the light together. For 

 example, iodide of silver is exposed to light in the presence of 

 pyrogallic acid, and nitrate of silver is reduced. If the iodide be 

 exposed separately, it is thrown into a state precisely similar to 

 that of a phosphorescent body, except that it sometimes continues 

 to vibrate in unison with the actinic instead of the illuminatiuor 

 raj's; and, so long as this condition remains, if it be brought in 

 contact with the above-mentioned substances, the effect is the same 

 as if they had been exposed together to ordinary light. 



For this property of light Mr. Lea proposes the name of Acti- 

 nescence, which has the merit of indicating the parallelism of the 

 phenomenon to that of phosphorescence. The faculty of receiving 

 a latent developable image depends, therefore, on two properties. 

 The first is that of being decomposed when brought in contact 

 with certain agents in the presence of light, that is, of being sensi- 

 tive to light. The second is that of being able to retain the influ- 

 ence of the chemical or actinic ra^^s, so that after the removal of 

 light, decomposition may be effected when in contact with those 

 certain agents alluded to, that is, the property of actinescense. 

 Mr. Lea's experiments and deductions are of great interest to those 

 who are studying photo-chemistry. 



SHOOTENG STARS. 



The November meteors have Ijeen the subject of several papers by 



European astronomers. Among the most important, are five let- 

 flxVST.] 40 



