256 



and the condition which makes this equal to zero, or the focus inde- 

 pendent of small variation of the refrangibility of the ray may be 

 satisfied, at the same time that the former equation is satisfied also ; 

 consequently, with three media, as in the eye, we may have perfect 

 achromatism for any two distances ; which would also be sensibly 

 perfect for the intervening ones. Of course by perfect achromatism, 

 we here mean a union of the extreme red and violet rays ; the irra- 

 tionality of dispersion does not concern this question." 



2. On the Intensity of Heat reflected from Glass. By Pro- 

 fessor J. D. Forbes. 



The author, after referring to a communication made to this So- 

 ciety, on the 18th March 1839, on this subject, and noticed in the 

 " Proceedings" of that date, stated, that being about to recommence 

 his observations on radiant heat, so long and unavoidably inter- 

 rupted, he had carefully examined the unpublished observations on 

 which the previous notice was founded, with a view to ascertain what 

 might be the numerical discrepancy which they present from Fresnel's 

 Theoretical Law. The variation in the results of experiment for 

 each of the angles was very considerable, arising from a multitude 

 of causes as yet imperfectly estimated, but which appear to have been 

 encountered by other observers, who, since that time, have undertaken 

 the same research. Under the circumstances, the mean of the whole 

 observations made between November 1838 and March 1839, have 

 been taken for each angle of incidence ; and the results being pro- 

 jected in the usual manner, the angles of incidence forming the line 

 of abscissae, and the intensities the oi'dinates, an interpolating curve 

 was drawn through the whole. The numbers thus obtained (which 

 are presented as only a rude first approximation), are shewn in the 

 following table, and compared with Fresnel's Formula, calculated for 

 an index of refraction of 1'50. 



Incidence. 



0° 

 10 

 20 

 30 



