364 E. B. Hunt on the Dispersion of Light. 



Art. XXXII. — On the Dispersion of Li^ht ; by Lieut. E. B. 



Hunt, U. S. Corps of Engineers. 



It is my purpose in this paper, 



I. To present a historical summary of the physical theories of 



chromatic dispersion. 



II. To prove by reference to certain astronomical facts, that 

 the unequal velocities of the different colored rays, forming the 

 basis of the present most received theories, cannot exist to the ex- 

 tent required. 



III. To present an outline of a new theory of chromatic and 

 thermotic dispersion, essentially different in principle from any 

 hitherto developed. 



The technical importance of the unequal refrangibility of dif- 

 ferent colored rays and their consequent dispersion in the prismatic 

 spectrum, is not less than its interest as a physical fact. To cor- 

 rect the chromatic observation and make a truly achromatic re- 

 fracting telescope and microscope, was a triumph as splendid in 

 results as difficult of attainment. The inability of the nndula- 

 tory theory clearly to explain this fact, has long been the most se- 

 rious objection to its unquestioned adoption, for it is charged not 

 with a mere failure to explain, but a seeming proof that the fact 

 cannot be. Nothing; brings us more in contact with the physical 

 character of light than the colored spectrum, and when we od- 

 serve that all nature is presenting in ceaseless variety these same 

 mingled hues, conversing with us almost solely through their 

 ministry, they gather a new and living interest. As to Joseph, 

 his "coat of many colors' 7 was an ever-speaking token ot that 

 paternal regard and fondness, which followed him in all his wan- 

 derings ; so to us does the " bow of promise" and the whole " shi- 

 ning robe of day." tell of a creating Father, and shadow forth 

 his kind control. 



I. Newton,* after experimentally proving that the different col- 

 ored rays have different refractive indices for the same medium, 

 and that these indices are independent of any mutual action of 

 the colored rays on each other, being constant for each in any 

 combination of rays or media, proceeded to show how, on the 

 emission theory, this unequal refrangibility or chromatic dispersion 

 could be explained by the received principles of mechanics. As- 

 cribing the whole fact of refraction to the action of molecular 

 forces at insensible intervals, he had only to conceive, in accord- 

 ance with observation, some cause of difference between colors 



p. 314. Wheweifs History 

 Treatise on Liybt : Art 564, 



tit -* 



■ rscbel" 



