382 



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 

 effected, — 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 oc- 

 casional changes, little differing from that of other experiments. 



4. This measure 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 cross bars 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 grey, 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 diflferent 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 viewed, 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 either 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 

 resulting spectrum, however, were not always synchronous portions, 

 sometimes appearing and disappearing by separate or partial changes, 

 like the effect of the dissolving views of the magic lantern. 



6. Pictures, diagrams, printing, &c., were found, under due influ- 

 ence of light, to ^aeld cognizable and sometimes vivid impressions 

 on the retina. Diagrams in black and white, or chequered surfaces 

 like that of a chess-board, gave very distinct pictures, always nega- 

 tive, the squares coming successively into view, beginning with the 

 portion gazed at. The succession of changes, when the impression 



