Royal Society. 545 



in optical spectra, embraced the results in respect of colour in the 

 images impressed on the retina, as derived simply from the influence 

 of light. 



The optical spectra from white, gray, or black opake objects under 

 faint illumination, or of ordinary windows or apertures transmitting 

 low degrees of light, were usually found to be without colour. But 

 ordinary daylight, and, much more, the light from bright sunshine 

 (as is well known), yield chromatic spectra of vivid or brilliant hues. 

 By viewing with slightly closed eyes, the pictures impressed on the 

 retina by a few seconds' steady gazing at some fixed point of an 

 illuminated object, and noting the various effects, disappearances and 

 changes, a considerable number of characteristic phaenomena were 

 elicited, and the effects of a variety of modifying circumstances satis- 

 factorily determined. The most prevailing influences in modifying 

 the phsenomena — whatever other causes might tend to the produc- 

 tion of variation in the colours — were found to be referable to differ- 

 ences in the degree of intensity of the external light, in the extent 

 of time occupied in gazing at the illuminated object, in the quantity 

 of light penetrating the chamber of the eye whilst examining the 

 spectra, and in the normal condition of the eye itself. These, with 

 other modifying circumstances, had been somewhat elaborately in- 

 vestigated. 



Different degrees of light, whether reflected from white objects, 

 or transmitted by colourless glass, had obviously the tendency to 

 yield differences in the colours of the primarily developed pictures 

 on the retina, with corresponding varieties in the nature and number 

 of the subsequent changes. Thus the viewing for a few seconds of 

 an aperture in a window the size of a pane of glass, whilst all the 

 rest was covered with a thick brown-paper screen, gave, with a low 

 degree of daylight, transparent pictures of a dingy orange, olive, yel- 

 low-gray or bluish black tint, changing, most usually, into a rusty- 

 tinted blackish spectrum, and disappearing, for the most part, in a 

 . minute of time or less. From medium degrees of daylight, the pri- 

 mary pictures embraced a considerable variety of colours, such as 

 crimson-pink, purple-pink, violet, purple, indigo, blue, — the blue 

 being the highest in the scale of intensity. The most marked 

 changes, commencing with blue, were usually from blue to red, or 

 to crimson, olive, black fading into blackish gray. In certain cases 

 rapid and evanescent glances were had of several intermediate colours. 

 The general photochromatic effects of the higher degrees of light, 

 such as from a clear sky in full sunshine, were far more uniform 

 than those from inferior light. The spectrum first elicited, even 

 after viewing a window or window-aperture for three or four seconds 

 only, was almost always green, with the character of illuminated 

 transparency ; the shades of colour however varied with the inten- 

 sity of the impression. The picture always appeared within four 

 or five seconds after closing the eyes, and when the light had been 

 strong and the gazing continued for a quarter of a minute or more, 

 the picture would burst out almost instantly. The restoration of 

 the picture in new colours, after the vanishing, had very much the 



