Intellisence and Miscellaneous Articles. 



215 



(Philosophical Magazine, Feb. 1843) on the different appearances 

 which the iodized plates assume when exposed for the same space 

 of time to varying intensity in the light, and submitted to the vapour 

 of mercury, attributed these effects to the unequal thick ness ot^the ' 

 substances deposited upon the plate s of silver, serv ing as reflecting 

 surfaces. He showed, moreover, that on operating upon paper 

 covered with iodide of silver, nothing was observed which indicated 

 the action o f negative rays, but tliat all t he active parts of the solaf 

 spect rum acted chemical ly in the same manner o n the iodide. 



The experiments which I have made on the chemical action of 

 the solar rays, from 1841 to 1844, have all led me to the same con- 

 clusion. Attention ought not therefore to be directed to the deposits 

 formed on the surface of Daguerreotype plates, as if these were the 

 only data to lead to the conclusion that the rays acted in various 

 ways ; for if so, thei^e would be risk of defective results. 



In support of the foregoing I will cite the following experiment, 

 of the accuracy of which any person will be able to judge. " Let a 

 Daguerreotype plate be prepared with iodine only (in order to avoid 

 the admixture of active substances), and let the blue, indigo, and 

 violet parts of a purified blue spectrum, presenting Fraunhofer'a 

 black lines, be thrown upon it. If the action only lasts a short time, 

 after submitting it to the mercury vapour, the black lines will be 

 seen to fix themselves upon a white ground, which represents those 

 parts affected by the active parts of the spectrum*." But if the plate 

 be exposed to the spectrum for an hour or more, then the appearance 

 of the plate changes, on passing it through the mercury vapour ; the 

 lines of the spectrum are scarcely marked, and the action has been 

 nearly uniform throughout its surface, but the lines whichare visible 

 appear w'hite, and show very distinctly upon the ground, which ap- 

 proaches to blackness : the effect is quite contrary to what it was 

 before. That portion of the plate which is acted upon by the violet 

 part of the spectrum has, under these circumstances, the same ap- 

 pearance as the portion of the plate exposed to the red rays by Messrs. 

 Foucault and Fizeau ; and to produce this eft'ect, it was only neces- 

 sary to vary the time of exposure of the plate to the same j)or- 

 tion of the spectrum. Must it be inferred, in the second case, that 

 the iodide of silver had been acted upon by negative rays ? Certainly 

 not ; for if the experiment be repeated uj)on iodide of silver laid upon 

 paper, the paper will become darker and darker, in proportion to the 

 time it continues exposed to the spectrum : and, besides, I have 

 fouud {Annales de C/iimie 'jt da Physique, J3rd series, vol. ix. p. '268 

 ct scij.) that the electrical effects arising from the chemical decom- 

 position of the iodide always act in the same direction. 



It will be seen that the conclusion to which Messrs. Foucault and 

 Fizeau came, viz. that tiiere exist in the red j)rismatic rays negative 

 rays, cannot be received, simply from the fact that the Daguerreo- 

 type plates are not always the same in aj)pearance. The contrary 

 eflects, as will be seen hereafter, are secondary effects produced by 



• K. l}ecc|iierel on the Constitution of the Solar Spectrum. — Bibliollieqiie 

 Univendle de Geneve, August 1842. 



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