NATURAL PHILOSOPHY. 177 



institution. The voltaic arc from metallic points furnished a spectrum 

 no less than six or eight times as long as the visible spectrum. This 

 was in fact the spectrum which had already been exhibited in connex- 

 ion with the fundamental experiment. The prisms and lens which 

 the lecturer had been employing in forming the spectrum were 

 actually made of quartz. The spectrum thus obtained was filled from 

 end to end with bright bands. When a piece of glass was interposed 

 in the path of the "incident rays, the length of the spectrum was 

 reduced to a small fraction of what it had been, all the more refracted 

 part being cut away. A strong discharge of a Leyden jar had been 

 found to give a spectrum at least as long as the former, but not, like it, 

 consisting of nothing but isolated bright bands. The lecturer then 

 explained the grounds on which he concluded that the end of the 

 solar spectrum on the more refrangible side had actually been reached, 

 no obstacle existing to the exhibition of rays still more refrangible, if 

 such were present. He stated also that during the winter, even when 

 the sun shone clearly, it was not possible to see so far as before. As 

 spring advanced he found the light continually improving, but still he 

 was not able to see so far as he had seen at the end of August. It 

 was plain that the earth's atmosphere was by no means transparent 

 with respect to the most refrangible of the rays belonging to the solar 

 spectrum. 



ON THE CHEMICAL ACTION OF SOLAR RADIATIONS. 



At the last meeting of the British Association, Mr. Robert Hunt 

 presented a report on the continuation of an examination of the 

 chemical action of the rays of the prismatic spectrum, after it had 

 been subjected to the absorptive influences of different colored media. 

 The mode of examination adopted has been to obtain well-defined 

 spectra of a beam of light passing through a fine vertical slit in a steel 

 plate by prisms of flint and crown glass and of quartz. The spectrum, 

 being concentrated by a lens, was received upon a white tablet and 

 submitted to careful admeasurement ; the colored screen (sometimes 

 colored glass and sometimes colored fluid) was then interposed, and 

 the alterations in the chromatic image were carefully noted ; the che- 

 mical preparation was then placed upon the tablet, and the chemical 

 impression obtained. The relation which this image bore to the lumi- 

 nous image was a true representation of the connexion between the 

 color of a ray and its power to produce chemical change. In the re- 

 port made to the Belfast meeting of the British Association, the results 

 of experiments made upon glass tablets prepared by the so-called col- 

 lodion process were alone given. In the present report, the examina- 

 tion has been extended to the photographic preparation known as the 

 calotype, and to iodide and bromide of silver in their pure states and 

 when excited by gallic acid. M. Edmond Becquerel, in a paper com- 

 municated to the Academy of Sciences, states " that when any part of 

 the luminous spectrum is absorbed or destroyed by any substance what- 

 ever, the part of the chemical rays of the same refrangibility is equally 



