218 Royal Society. 



the ultimate particles, is consequently much more difficult than 

 with crystals such as Mausite. Here, notwithstanding the di- 

 chromatism, one body alone, peroxide of iron, suffices ; for it is 

 sufficient to assume that the particles are so arranged, that when 

 a ray passes through the hexahcdral prism from surface to sur- 

 face, becoming thereby divided into two polarized perpendicular 

 to one another, it meets a greater number of material points or. 

 particles with the vibrations of the ordinary ray perpendicular to 

 the axis, than with those of the extraordinary ray in the direction 

 of the axis. 



Hence the differences of tint, even the olive-green, which in 

 comparison with hyacinth-red appears still more green, present 

 no difficulty to this explanation. They depend simply upon 

 greater or less absorption. On this account, however, the colours 

 of crystals which contain peroxide of iron in combination with 

 oxalic acid and alkalies appear all the more enigmatical ; for 

 even when they are dichromatic, the most beautiful grass-green 

 is coexistent only with quite pale, yellowish-white tints, which 

 at furthest reach but to olive-green. 



XXXIII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 149.] 



Dec. 22, 1853.— Thomas Graham, Esq., V.P., in the Chair. 



HE following paper was read :- 



T 



" An Inquiry into some of the circumstances and principles 

 which regulate the production of Pictures on the Retina of the Hu- 

 man Eye, with their measure of endurance, their Colours and 

 Changes." By the Rev. W. Scoresby, F.R.S., Corresponding Mem- 

 ber of the Institute of France, &c. 



The investigations of the author embrace three distinct cases, — 

 the case of achromatic pictures ; that of coloured jjictures of un- 

 coloured objects, derived simpl)^ or mainly from the influence of light 

 on the eye ; and that of the spectra of coloured objects, together 

 with certain ajiplications of the results obtained to other optical 

 characteristics, determinations or phsenomena. 



The general mode of experiment employed in these researches is 

 described as " the viewing of illuminated objects with a steady fixed 

 gaze at a special point, and then determining the impression on the 

 retina by exaiiiining the images developed with closed eyes." The 

 time of viewing the objects varied from a momentary glance up to 

 half a minute, more rarely to a minute ; and the mode of eliciting 

 the impression was, primarily, by closing the eyelids into gentlest 

 contact, whilst the head was kept unmoved, and the eyelids steady 



