NATURAL PHILOSOPHY. 141 



spirit-lamp. The plates thus prepared may be preserved in the dark 

 for a long 1 time. In diffused light, the surface of chloride of silver, 

 thus prepared, becomes gray ; but if \ve project a very pure and con- 

 centrated prismatic spectrum, it recehes, at different rates, impressions 

 from all the visible luminous rays in their respective colors, at the 

 same time that very decided colors are produced by the non-luminous 

 rays below the red and beyond the uolet. By warming the prepared 

 plate some curious changes are produced ; and if warmed on a stove 

 to about 212 Fahrenheit, M. Becquerel states that the most perfect 

 condition for imprinting the spectral colors is brought about. The 

 time which the plate should be exposed to the solar spectrum varies 

 with its intensity ; when very concentrated, in a few minutes a fine- 

 ly colored impression is obtained. These photographic images may 

 be preserved for a considerable time in the dark-; but as yet no means 

 have been discovered by which they can be rendered permanent 

 against the continued action of light. 



Admiring the zeal with which M. Becquerel has pursued his re- 

 searches on this curious subject, we must not forget that Sir John 

 Herschel has also succeeded in obtaining a colored impression of the 

 spectrum, on paper prepared with vegetable juice ] and that Mr. R. 

 Hunt got a similar result with fluoride of silver. We may, therefore, 

 reasonably hope that eventually the pencil of the sunbeam will add the 

 charm of color to the chemical pictures it produces. London Athenccum. 



LUNAR DAGUERREOTYPES. 



THE existence of actinism, or the chemical principle of light, in the 

 rays reflected from the moon's surface, has heretofore been a question 

 of considerable doubt and uncertainty. At a meeting of the British 

 Association, at Cork, some years since, Dr. Robinson stated, " that 

 he had been led by the success of Professor Rondoni of Rome, in 

 procuring Daguerreotype images of various fixed stars and nebulas by 

 means of light transmitted from these objects, to endeavour to procure 

 a Daguerreotype impression of the moon's surface. A portion of the 

 disk of the moon was brought within the range of a powerful reflect- 

 ing telescope, and the brilliant image formed thrown upon a Daguerre- 

 otype plate placed in the focus of the reflector. The plate was left 

 exposed in this situation for twenty minutes. Although a good im- 

 pre^sion of a building could be procured upon plates similarly pre- 

 pared, in a minute, yet this prolonged exposure to the light of the 

 moon produced no impression." Dr. Robinson considered the ex- 

 periment as conclusive in establishing the fact, that the chemically 

 active principle known as actinism did not exist in lunar light. Re- 

 sults similar to those of Dr. Robinson, have been also arrived at by 

 various philosophers in Europe and America. Dr. Draper, of New 

 V''.rk, however, has stated that he has been able to detect the ac- 

 tinic element both in moonlight and artificial light. 



At a meeting of the Cainbridi. 1 ' 1 Scientific Association, December, 

 1849, five Daguerreotype pictures of the moon's surface were exhibited 

 to the Society by Mr. Wells. These pictures were taken by Mr. 



