50 ACTION OF THE SPECTRUM OF THE DAGUERREOTYPE PLATE. 



exchanged for a perfect whiteness, marking by a clear and sharp outline the lateral ex- 

 tent of the calorific rays, and by due gradations of intensity in a longitudinal direction 

 their law or scale of distribution, both within and without the luminous spectrum." 



172. " The most singular and striking phenomenon exhibited in the thermic spec- 

 trum thus visibly impressed is its want of continuity." It consists of several distinct 

 patches, or round solar spots, as is seen \njig. 119. 



173. These are the leading facts in relation to the distribution of heat in the New- 

 tonian spectrum. Let Us, in the next place, proceed to examine the laws of the dis- 

 tribution and action of the chemical rays, commencing first with the decomposition of 

 one of the salts of silver, as, for example, the iodide of silver, which forms the basis of 

 the Daguerreotype. 



174. The action of the Newtonian spectrum on the Daguerreotype plate is very re- 

 markable. Imperfect attempts were made, soon after the preparation was published, 

 to determine the changes which happen to this very sensitive substance. For the most 

 part, these, however, were made without any kind of precaution as respects the purity 

 of the dispersed colours precautions which NEWTON, a century ago, so clearly pointed 

 out. Instead, therefore, of spectra of great purity, in which the length is many times 

 greater than the breadth, drawings were given in which the breadth of the solar image 

 is equal to its length. Whoever remembers that, in the experiments conducted by NEW- 

 TON with a view of determining the phenomena of homogeneous light, and in which 

 the lengths of his spectra were sometimes seventy times greater than their. breadth, will 

 easily understand how small must be the value of such imperfect proofs, which, indeed, 

 can scarcely be called impressions of the solar spectrum. 



175. The impressions obtained in July, 1842, in Virginia (Ap., 645647), and which 

 may be taken as almost perfect specimens of the action of this variety of spectrum, 

 were made by strictly following the precautions of NEWTON (152, 153). Several of 

 them were obtained, representing the action under various changes in the dimensions 

 and figure of the aperture admitting the light ; as, for example, when it was a circle, a 

 triangle, a fissure. Of the former of these, a very elaborate account has been given in 

 the Philosophical Magazine, by Sir J. HERSCHEL (Phil. Mag., Feb., 1843), who has 

 deduced from it what is probably the true theory of the action of light on the Daguer- 

 reotype plate. In these perfect spectra there are certain peculiarities which cannot at 

 all times be produced ; they require an exceedingly brilliant sun, and a clear sky. In 

 tropical latitudes there is no difficulty, during the heat of summer, in obtaining them ; 

 but in New- York, during a great many trials which I have made in the course of two 

 years, I have but once been able to reproduce them. 



176. The solar spectrum obtained upon iodide of silver may be roughly divided into 

 two great and almost equal regions : the upper region, corresponding to the more re- 

 frangible rays, embraced between the blue and the extreme violet, exhibits, by a deep 

 dark colour, or series of colours, that a decomposition of the iodide has taken place; 

 probably in those portions in which this decomposition has been complete, the iodide 

 has lost one half of its iodine; the lower region, which is by far the most interesting 

 portion of the two, and by far the most difficult to study, corresponds to the lesser re- 



