70 REPORT — 1853. 



(76) 65. The chemical change is confined to a space 



between the mean yellow ray of the spectrum, near the Fig- 38. 



line E, or "25 above a', and the upper verge of the green ray, 2_ 



or "70 above a' ; the oval being usually from '42 to "45 in 



length, a narrow neck extending upwards. Upon close ex- j, 



amination, it is apparent that, even in this shortened chemical || 



spectrum, we obtain indications of the two actions already >^ 



described. m| 



(77) 66. Pure bright yellow, by Silver By this w^ 



glass the blue rays ai-e completely obliterated, and the green ^^ 



rays somewhat shortened ; but all the least refrangible rays 

 are preserved in their purity, with the exception that the orange rays are some- 

 what reduced, and appear as a well-defined band, of more brilliancy than when 

 seen without the interposition of this medium ; and much white light is seen 

 ill the yellow ray. In observing natural colours through this glass, the 

 absorption of all the blue rays, and those beyond, becomes very sensible. 



(78) 66. In this case the chemical change occurs over the space covered 

 by the most luminous rays, the orange, yellow, and the least refrangible green. 

 It is comprehended within a space equal relatively to the cor- Fig. 39. 



rected length of the spectrum of about one-third of an inch, «_ 



and within these limits are discoverable three defined ac- 

 tions, differing in the intensity of the effect produced, and 



of the resulting colour of the impression ; the lower space Mik 

 corresponding with the upper orange ray being several ||||| 

 shades lighter than that darkened by the yellow ray, and WW 

 again, the action of the green is far less intense. Where the 

 gradations of shade are very slight, it is not easy to speak ^■ 



decidedly as to their character ; but the least refrangible 

 space may be described as gray ; the next in order, and by far the most in- 

 tense, as an iron-gray or brotize; and the next as a pure broivn. There were 

 sometimes indications obtained of a central line of action, extending from the: 

 green up into the blue rays ; but this was always exceedingly faint, and only 

 to be found when the atmosphere was clear and the sun very bright. 



(79) 15. Straw-yellow. Silver stain upon one surface only. (Par. 5, 

 First Report, 1852.) — It will be seen, by reference, that this glass cuts off a 

 considerable portion of the violet rays, leaving the other rays without any 

 considerable change. 



An impression of this spectrum on collodion was not obtained, therefore 

 the present one on iodized paper is not comparable with any previous im- 

 pression. 



(80) 15. Action commences at '20 above a!, and extends Fig. 40. 

 over the more luminous space with the greatest intensity ; 

 then the action suddenly weakens over the limits of the 

 green rays, growing more intense under the action of the ~ 

 blue and indigo rays. A still more decided weakening of 

 chemical activity occurs at about -18 below a, from which 

 space unto -20 beyond a, a faint indication of action is con- 

 tinued. Thus we have here two very remarkable maxima 

 and minima; the former in the yellow and blue rays, and 

 the latter in the green and violet rays, and beyond them. 



(81) 16. Deep xellow, Carbon. — For chromatic ana — 

 lysis see Par. 4, First Report ; and for chemical action. Par. 

 51 ; the preparation then employed being a highly-sensitive collodioa'i 

 plate. 



