• 



76 REPORT 1653. 



undergone absorption and dispersion by the uranium glass is not a little re- 

 markable, and requires to be studied with much care. Before each experi- 

 ment with this glass, it was my practice to obtain an impression from a very 

 pure concentrated spectrum which had not been subjected to any absorption ; 

 the object of this being to determine exactly the relation which the chemical 

 spectrum after absorption and dispersion bore to the unabsorbed image. 

 This was necessary, as it was found there were many variations, from day to 

 day, in the chemical powers of the several spaces corresponding with the 

 coloured rays. Under all circumstances there was the same general character 

 in this impressed spectrum pj 54 



after absorption as in many 



of those already described. . . 



The action was divided A jm. 



intotwowell-definedspaces. ji WR 



The rays which are che- "" ^^ 



mically active from the 



mean yellow rays up to the 

 blue produce a well-defined 

 image varying in inten- 

 sity : first a dark-olive 

 oval, and above this we 

 have a second brown oval. 

 This, however, stops short 



of the end of the visible ^- 



spectrum, terminating in the mean violet ray. Measurement according to 

 the scale I have adopted throughout gives the following result. The first 

 image in the figure represents the normal chemical spectrum : — 



First indication of chemical action above a' '15 



Point of termination of the first oval „ '55 



Commencement of second oval „ '50 



Termination of second oval „ '90 



Entire length of the image formed within the limits ofl __ 

 the visible spectrum j 



A space without any chemical change, equal to '45, then occurs. This ap- 

 pears to agree with the extreme violet ray and the lavender ray of Sir John 

 Herschel, and the lines of Becquerel and Stokes beyond. Beyond this, that 

 is, at •45 above a', the most refrangible limits of the known spectrum, a third 

 oval forms, the entire length of which is -iO ; so that the whole length which 

 undergoes chemical change is 'US, with the interruption of the action above 

 H to about Mr. Stokes's lines /. If the spectrum is made to pass through 

 the width of the block of uranium glass, which is 2^ inches, the action 

 beyond the spectrum is entirely obstructed ; but over the space covered by 

 the most luminous rays chemical action goes on, with an intensity nearly 

 equal to that whiclf is produced when the thickness of the slab about 1 inch 

 only is used for absorption. The third figure in the woodcut represents the 

 result which is obtained. 



(109) 79. Bromide of Silver instead of the iodide of silver was employed. 

 The differences are not remarkable between those impressions and those 

 already described, as being obtained on the iodized paper. The first oval 

 of the spectrum after absorption descends rather lower, and shows a very 

 decided action, due to the yellow rays, superior to that shown when the iodide 

 is employed. When the rays are made to pass through 2^ inches of the 



