JOHN WII.i.ia.M DBAPBB. 



that Dr. Diaper, in 1843, discovered the dark lines «, ,'?, ,- in the 

 ultra-red. Essentially the same results are obtained if the plate if 

 labmitted to a weak light for i few momenta previous to its ex- 

 posure to the spectrum. It would seem, therefore, as if the less re- 

 frangible rays could reverse the general action of light upon iodide 

 of silver. So that, while every ray of the spectrum is capable of affect- 

 ing this substance, the more refrangible promote, the less refrangible 

 arrest, this general action of light upon it. 



Bitumen and resins also receive impressions from below A to be- 

 yond H, every ray in the spectrum acting. In the bleaching of 

 flowed by light "the rays which are effective in the destruction of 

 any one vegetable color are precisely those which by their union 

 produce a tint complementary to the color destroyed." Chlorine 

 and hydrogen unite under the action of the indigo ray, which is seven 

 hundred times more active than the ultra-red. Dr. Draper con- 

 cludes, therefore, "that the sensitiveness of any given preparation 

 to light depends on its chemical nature and its optical qualities con- 

 jointly, and that it is possible to exalt or diminish the sensitiveness 

 of a given compound by changing its optical relations." 



The results of Dr. Draper's investigations with the prismatic 

 spectrum, however, were not altogether satisfactory to him, owing 

 to a defect which originates in the very cause which gives rise to the 

 spectrum itself — unequal refrangihility. If we compare together 

 two sets of rays, one taken in the red and the other in the violet 

 region, it is obvious that, in the same spectrum, from the very cir- 

 cumstance of their greater refrangihility, those in the violet will be 

 relatively more separated from each other than those in the red. 

 The result of this increased separation in the more refrangible 

 regions is to give an apparent dilution to them, while the less re- 

 frangible regions are concentrated. 



Accordingly, in May, 1843, Dr. Draper succeeded in inducing 

 Joseph Saxton, the eminent mechanician of the United States mint 

 at Philadelphia, to rule for him a diffraction grating. With this 

 grating, which was of glass and five-eighths of an inch by one-third 

 of an inch in size, the diffraction spectrum was produced and the 

 above difficulties avoided. Dr. Draper effected a great improve- 

 ment in the grating by silvering its ruled surface with tin amalgam, 

 thus producing a reflected spectrum which was far more brilliant 

 than the transmitted one, though he suggested that perhaps it would 

 be better to rule them originally on steel or speculum metal. It 



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