THE 



LONDON AND EDINBURGH 

 PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



MARCH 1835. 



XXV. Observatio?ts on the supposed Achromatism of the Eye. 

 By Sir David Brewster, F.R.S., &c. 



¥N a paper " On the Achromatism of the Eye,'^ by the Rev. 

 ■■■ Baden Powell, just published by the Ashmolean Society 

 of Oxford, he has endeavoured to refute the opinions and 

 statements of different authors who have maintained that the 

 eye is not an achromatic instrument. As Mr. Powell has re- 

 ferred to opinions and experiments of mine upon this subject, 

 I feel myself called upon either to renounce them, if they are 

 wrong, or to endeavour to explain and support them if they 

 are correct 



^ The experiment on the marginal dispersion of my own eye, 

 quoted by Mr. Powell, is admitted by him to be so far de- 

 cisive of the question as to prove " that the principle of its 

 achromatism (if it exist) must be such as is not effective in ob- 

 lique excentrical pencils^^ p» 1 1 ; but he is of opinion, that 

 notwithstanding this, the eye may be in general achromatic 

 for direct rays. 



In order to establish this opinion, Mr. Powell discusses 

 some very decisive experiments of Fraunhofer, which I pub- 

 lished in the Edinb. Phil, Journal, No. xix. p. 35. The objec- 

 tions which he makes to these experiments do not, in my opi- 

 nion, invalidate the results which their illustrious author de- 

 duced from them ; and I have every confidence in the con- 

 clusion at which he arrives, that, in his eye, blue rays must 

 diverge from a point 21*1* inches distant, in order to have 



* This is the mean of four experiments. 

 Tfiird Series, Vol. 6. No. 3S. Mar. 1835. Y 



