NATURAL PHILOSOPHY. 137 



be induced by exposure to daylight for a few seconds, or to the 

 light of a piece of magnesium wire. 



IMPROVEMENT IN OBJECTIVES. 



At a recent meeting of the Massachusetts Institute of Technol- 

 ogy, Mr. Charles Stodder exhibited the immersion lenses of Hart- 

 nack, and similar ones made by Mr. Tolles, of Boston. He ex- 

 hibited one of Mr. Tolles' one-fifth objectives, of only half the 

 power of Hartnack's, which resolved lines onNobert's test, 112,000 

 to the inch, a microscopic excellence hitherto unparalleled in the 

 records of the science, and proving the extraordinary superiority 

 of Mr. Tolles' lenses. 



ON THE NATURE OF THE LATENT IMAGE IN PHOTOGRAPHY. 



When light, considered simply in reference to its illuminating 

 power, falls upon any substance, we are accustomed to consider 

 the effects of that illumination as passing away at the same instant 

 of time that the illumination terminates. But there are a vast 

 number of well-recognized exceptions to this rule, which we know 

 under the names of phosphorescence and fluorescence. 



If certain bodies, known as "phosphorescent," be exposed to 

 a bright light, such as the direct rays of the sun, and then be re- 

 moved to the dark, they will emit a very distinct light. This light 

 continues to be emitted for a time of variable duration. With 

 some substances it continues for days ; with others it terminates in, 

 a few hours. Becquerel has enormously extended the number of 

 substances that act in this way, by showing that the period of time 

 during which they phosphoresce may be exceedingly short, and 

 so escape ordinary observation. He constructed an extremely in- 

 genious instrument by which phosphorescence could be made 

 evident even when it continued for but a very minute fraction of a 

 second after the light which fell upon the substance was removed. 

 These facts, then, embraced under the general term of phospho- 

 rescence, prove incontestably that bodies may, by light, be thrown 

 into a state of vibratory motion, lasting for a longer or shorter, 

 sometimes a very considerable, time after the exciting cause is 

 removed, and that, so long as this vibratory movement continues, 

 they will themselves emit light. 



But light, such as it comes to us from the sun, is endowed with 

 another property distinct from illumination, and which we con- 

 veniently term actinism. There is not the slightest reason to doubt 

 that bodies may be endowed with the power of being impressed 

 by these rays, and retaining them precisely as bodies may the il- 

 luminating rays. Herein lies the explanation of the physical or 

 late n't image. It is simply a phosphorescence of actinic rays. 

 Once stated, the whole matter is so evident as to carry conviction 

 with the simple statement. 



Lot me, then, explain the manner in which this phenomenon 

 takes place with iodide of silver. Pure iodide of silver under- 

 goes no decomposition by light when thoroughly isolated from 

 all substances, organic and inorganic, which are capable of aiding 



12* 



