NATURAL PHILOSOPHY. 145 



immediately resume the form of the inscribed octahedron. In addition to 

 these forms the instrument also can be made to demonstrate the passage of 

 all the hemiliedral forms of the cubical system, with inclined faces within 

 their limits. In this manner, it was demonstrated that this instrument can 

 make visible to the eye all the changes and varieties of an interesting series 

 of forms and their mutual relations, which could otherwise only be conceived 

 by a considerable power of mathematical abstraction. This armillary sphere, 

 by some other small additions, can be make use of for tracing out some of 

 the most beautiful portions of the zone-theory of the poles of crystals. Rev. 

 Walter Mitchell, M. A., Proc. Royal Institution, London, March 18, 18-39. 



OBSERVATIONS ON PHOSPHORESCENCE. 



At a late meeting of the American Photographic Society, New York, the 

 subject of phosphorescence being under discussion, the President, Professor 

 W. H. Draper, made the following remarks : " I will mention a fact or two 

 which may be found to have some bearing on the question of stored-up 

 light. If the powder of sulphide of calcium be spread on some convenient 

 surface, as a sheet of tin, and upon this a key be laid, and the whole be ex- 

 posed for a few minutes to the sunlight, on bringing it in a dark room and 

 removing the key, the whole surface will shine, except where the key left its 

 shadow. The image of the key will appear black on a white ground. The 

 phosphorescent light, however, gradually diminishes, till the image of the 

 key cannot be distinguished. If now a ring be laid on the powder, and the 

 surface be again exposed to sunlight, in the dark the image of the ring will 

 appear and disappear. The experiment may be continued with other objects, 

 and with precisely similar results. So far you find nothing that you did not 

 khow or might easily anticipate. But now heat the plate in the dark, and 

 the images of the key and ring, and other objects, will reappear. These 

 images were impressed, for a considerable time were latent, and again they 

 are developed. A phosphorescent which has lost its power to shine in the 

 dark, recovers this power when, a spark of electricity is sent through it. 

 The light now given out passes readily through quartz, while glass is opaque 

 to it. I have examined a great many diamonds in the study of phosphor- 

 escence. I have observed that yellow diamonds are invariably phosphores- 

 cent, and shine with brighter light than others. If, after the diamond has 

 ceased to shine in the dark, it be warmed in the hand, it glows again, but 

 only for a short time; this property may be restored successively at increas- 

 ing temperatures." 



ON PHOSPHORESCENCE, FLUORESCENCE, ETC. 



The following is a report of a lecture on the above subject, given by Pro- 

 fessor Faraday, before the Royal Institution, June 17th, 18o9: The agent 

 understood by the word " light," presents phenomena so varied in kind, and 

 is excited to sensible action by such different causes, acting apparently by 

 methods differing greatly in their physical nature, that it excites the hopes 

 of the philosopher much in relation to the connection which exists between 

 all the physical forces, and the expectation that that connection may be 

 greatly developed by its means. This consideration, with the great advance 

 in the experimental part of the subject which has recently been made by E. 

 Becquerel, were the determining causes of the production of this subject 

 before the Members of the Royal Institution on the present occasion. The 



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