Prof. Powell on the Achromatism of the Eye, 24<7 



heat, have not, in fact, a certain small amount of it, very de- 

 licate thermometers will be required. These instruments must 

 be susceptible of indicating variations of temperature equiva- 

 lent only to small fractions of a degree on any of the scales at 

 present in use, as the differences of temperature of nearly 

 allied species, in the one line of research, and those between 

 the animal and the ambient medium, in the other, must un- 

 questionably be very minute. 



A popular exposition of this subject, including the enuncia- 

 tion of the approximate law above explained, was delivered, 

 in the form of a Lecture, in the Theatre of the London Insti- 

 tution, to the company assembled at a Conversazione on the 

 27th of February last (1833), on which occasion the foregoing 

 views were for the first time explicitly announced. 



London Institution, June 19, 1833. 



XL I. On the Achromatism of' the Eye : in reply to some Re- 

 marks in the Lond. and Edinh. Phil, Mag. and Journal of 

 Science, No, 33. By the Rev, B. Powell, M.A., F.R.S., 

 Savilian Professor of Geometry, Oxford,"^ 



/RESERVING in the last Number of this Journal (p. 161) 

 ^^ that Sir David Brewster has honoured my paper on the 

 achromatism of the eye with a notice, I think it necessary, in 

 order to prevent misconception, to offer the following brief 

 statement in reply. 



1st. I believe I have stated in my paper, that I readily ad- 

 mit that many individual eyes may not he achromatic. If the 

 dispersive powers of the media vary ever so little, the achro- 

 matic adjustment may be destroyed. lam therefore far from 

 wishing to controvert any particular observations which may 

 show that individual eyes have not the requisite adjustment. 



2ndly. The main object of my paper was to show, in con- 

 tradiction to the assertions of many theoretical writers, that 

 in a comhinatio7i similar to the eye, achromatism is perfectly 

 possible in principle. Their position is that the result is im- 

 possible upon theoretical and mathematical grounds. 1 have 

 shown in a way which can only be refuted by disproving 

 the whole established theory of foci and refractive indices, 

 that, as far as theory is concerned, achromatism is perfectly 

 obtained, in a combination of a lens and one medium, if only 

 the indices and radii fulfill the conditions of a certain formula. 

 I have also shown by observation, that in the particular in- 



* Communicated by the Author. 



