Contributions to tfie Physiology of Vision. 113 



the first two images of the flame are the same, but the former 

 white spectrum is now of a dark grey colour with a white 

 margin. The same appearances are obtained when the flame 

 has been regarded for a longer time, except that the metamor- 

 phoses take place more slowly, and the proportion between the 

 time during which the flame has been looked at, and the dura- 

 tion of the spectrum remains always the same, viz., about 

 1 :20. 



2. When the flame has been stedfastly looked at for a much 

 longer time (from twelve seconds to a minute) the succession 

 of the images is nearly the same as in the first experiment, 

 except that the bright and coloured ones rather predominate. 

 First the bright image of the flame is seen, then the yellow, 

 red, blue, white, and black images follow, and the whole 

 appearance is ultimately covered by the grey gleam, as in the 

 above case. All the images disappear in a centripetal direc- 

 tion, the first much more rapidly than the others, the black 

 remaining visible for the longest time. During the experi- 

 ment, fragments of the vascular figure are frequently observed ; 

 they are of the same colour as the image in which they are 

 perceived. 



3. If the sun or the focus of a lens has been stedfastly 

 regarded for a short time, a bright white image remains, and 

 lasts for a considerable time ; the coloured spectra then appear, 

 and follow each other in rapid succession. 



4. If the windows be regarded on a cloudy day for about 

 twenty seconds, and the eyes be then quickly covered, at first 

 the panes are seen white and the frame black, but the former 

 rapidly change into black and the latter into white ; and after 

 a repetition of these changes four or five times, the whole 

 appearance is dissolved in a grey gleam. 



IX. Very different from the ocular spectra are what might 

 perhaps be called the mental spectra, viz., the images of objects 

 before the internal eye, (if the expression may be allowed,) 

 after they are inaccessible to the external sense, as for in- 

 stance, during winking, or whilst shutting the eyes during 

 meditation. They seem to depend entirely on the observer's 

 will and attention, and may even be recalled after having com- 



VOL. I. OCT. 1830. I 



