OF ARTS AND SCIENCES : APRIL 14, 1868. 



481 



a Minimum of Dispersion. 



? Minimum of Deviation. 



then the other, the second will neutralize the dispersion of the first, 

 while there will remain 17° 53' — 11° 7' = 6° 46' deviation. It was 

 by this arrangement of prisms that Brewster obtained achromatism with 

 one kind of glass. 



Perhaps the most important application of these principles is to pho- 

 tographing the spectrum. Photographs taken by the common methods 

 are greatly distorted, particularly in the more refrangible end. They 



61 



VOL. VII. 



