220 Royal Society. 



such spectra were yielded by extremely low degrees of illumination. 

 The light, for instance, of the moon or stars thrown on a white 

 linen blind, produced distinct negative pictures of the slightly illu- 

 minated aperture. Candlelight gave also negative pictures of white 

 and black objects. Low illumination from transmitted solar light 

 gave, in most cases, colourless pictures, appearing sometimes imme- 

 diately on closing the eyes, as by a flash of light, or otherwise in 3 

 to 5 seconds in negative tints ; these pictures, where the object had 

 been viewed for some seconds, were found to fade away and subse- 

 quently reappear in less dark shades, sometimes with several such 

 changes. 



3. The changes in the optical spectra from the partial or entire 

 exclusion of light from the closed eyelids were found to be very 

 striking. No matter how this diminution or exclusion of light was 

 eifected, — whether by the thickening of the eyelids by compression, 

 or turning the face away from the light, or interposing the hand 

 or other opake substance betwixt the eyes and the light, or covering 

 the face altogether, — the spectra assumed a new character as to light 

 and shadow, ordinarily, but not in all cases, complementary to the 

 tints originally observed. A total exclusion of the external light 

 still left the picture clear and distinct, with a continuance, after 

 occasional changes, little differing from that of other experi- 

 ments. 



4. This measui'e of fixidity of the spectra impressed on the retina 

 led the author to some curious results in obtaining duplicate or 

 multiple pictures of the same object. Thus, by gazing at a window, 

 successively at different fixed points previously determined on, he 

 multiplied the crossbars so as to produce a picture of a window with 

 twice or quadruple the number of panes. A white statuette, viewed 

 at different points in succession, whilst strongly illuminated, enabled 

 the author to obtain double pictures in black or gray, associated 

 according to the relation of the points gazed at, in unlimited variety. 

 Or viewing the statuette from two positions differing in distance, he 

 obtained images of different dimensions. Double images, too, were 

 obtained by using the eyes separately ; and also by looking at an 

 object nearer to the eyes than the statuette, so that the lines of the 

 axes might diverge at the distance of the statuette, thus beautifully 

 elucidating one of the chief causes of the indistinctness of vision as 

 to objects nearer to, or more remote from the eyes than that directly 

 contemplated. 



5. Complete pictures were also obtained by the combination of 

 parts separately %'iewed, whilst various impressions, however incon- 

 gruous, were combined into one picture. Thus parts of the statuette 

 were viewed, under the adoption of a moveable screen, so as either 

 to combine the separately-viewed portions rightly, or to transfer one 

 part, such as the head, to cither shoulder, or to adjust two heads in 

 different positions. Separate impressions, also, of segments of the 

 statuette were taken on the eyes singly, and these combined, ac- 

 cordingly as the same or different points of view were selected, 

 into perfect or distorted pictures. The appearance of the parts of the 



