of the Negative or Protecting Rays of the Sun. 89 



The observations I made in Virginia were as follows. That 

 if a solar spectrum was received on a Daguerreotype plate on 

 which a weak daylight was simultaneously acting, the red, 

 orange, yellow, green, and part of the blue rays, arrested the 

 action of the daylight on that portion of the plate on which 

 they fell, and maintained it in the unaffected state ; whilst the 

 residue of the blue, the indigo and violet, carried their part of 

 the plate to a completely solarized condition. This therefore 

 seemed to justify the assertion, that the less refrangible rays 

 protect Daguerre's preparation from the action of a diffused 

 daylight. 



It was also found that if the plate was exposed to the day- 

 light for a few seconds, so that had it been then mercurialized 

 it would have whitened uniformly all over, on being made to 

 receive the spectrum the less refrangible rays actually carried 

 it back to the unaffected condition, reversing what had been 

 already done. Whilst the more refrangible rays were forcing 

 it on to the solarized state, these were returning it into the 

 condition of shadow : they therefore not onXy protect, but seem 

 even to exert a negative or antagonistic action. 



The following observation appears to me to be in contrast 

 with the beautiful explanation which Sir J. Herschel gives of 

 the phaenomena of one of these specimens, and moreover to 

 have a very significant meaning. I found that it was perfectly 

 immaterial whether the exposure to the spectrum was for thirty 

 seconds or one hour — the result was the same. The final 

 action had been produced, the less refrangible rays had car- 

 ried their region to the unaffected state, while the more re- 

 frangible had solarized theirs. Now if the phaenomenon was 

 due, as M. Becquerel supposes, to an unequal action of the 

 same kind in the different rays, it is obvious that the final re- 

 sult ought to depend on the time of exposure ; the red ray, 

 aided by the daylight, should carry its portion through the 

 various shades of white, and solarize it at last. But this in 

 the longest exposure never takes place ; that part of the plate 

 remains as though a ray of light had never fallen upon it. 



Such are the facts I observed, and they seem to have been 

 reproduced by MM. Foucault and Fizeau ; but there are also 

 others of a much more singular nature. In these Virginia 

 specimens the same j^i'otcctijig action reappears beyond the 

 violet. 



The only impressions in which I have ever seen this pro- 

 tecting action beyond the violet, are those made in Virginia in 

 1 81-2 ; they were made in the month of July. Struck with this 

 peculiarity, on my return to New York the following August I 

 made many attempts to obtain similar specimens, but in no in- 



Fhil. Mag, S. 3. Vol. 30. No. 199. Feb. 1847. H 



