118 



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-grey 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 grey. 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 

 character and appearance of the dissolving views effected by the magic 

 lanthorn. The frame of the window or aperture, and the cross- 

 bars, were always pictured in colours different from those of the 

 panes, besides a fine marginal line of another colour dividing the 



