548 FOURTH REPORT — 1834. 



white will acquire a tinge of yellow ; if the blue and green be 

 successively stopped, this yellow will grow more and more ruddy, 

 and pass through orange to scarlet and blood red. If, on the 

 other hand, the red end of the spectrum be stopped, and more 

 and more of the less refrangible portion thus successively abs- 

 tracted from the beam, the white will pass first into pale, and 

 then to vivid green, blue-green, blue, and finally into violet. If 

 the middle portion of the spectrum be intercepted, the remain- 

 ing rays, concentrated, produce various shades of purple, crim- 

 son, or plum-colour." 



The subject of this paper admits of more lengthened and ac- 

 curate treatment than is given to it here. The object of this 

 communication is merely to call attention to a circumstance 

 which appears to have been overlooked in the undulatory 

 theory of light, viz. an analogy existing hetiveen the composi- 

 tion of colours atid the composition oj" small vibrations. 



On the Achromatism of the Eye ; in contimtatio?t of a Paper 

 in the last Volume of the British Association Reports. By 

 the Rev. Baden Powell, M.A., F.R.S., Sav. Prof, of Geo- 

 metry, Oxford. 



In the paper referred to the author inadvertently introduced 

 a formula which he did not observe was incorrect till the sheet 

 had been printed. The correct expression will be found by 

 taking the genei'al formula for the principal focal length (F) 

 after refraction through two surfaces, at which the relative in- 

 dices (taking the sines in the order of transmission) are /u, ju., 

 and the radii r r (remaining to be affected by their proper 

 signs), which is (see the author's Optics, p. 23,) 



1 ^ I ^,-1 ^ 1 f^-1 

 F r, [x,, r ij^ii^ 



Adapting this to a double convex lens (when r becomes nega- 

 tive), and equating similar expressions for the red and violet 

 rays, the condition of achromatism will be found to be 



fV _ [(y + r,) f^r - rj] fi.„ 



f^iv [('• + '•;) l^v - r,] l/^r 



When this is fulfilled, achromatism may be pi*oduced by the 

 nature of the medium in which the focus is formed. This prac- 

 tically differs little from what was given in the former paper. 



