172 ANNUAL OF SCIENTIFIC DISCOVERY. 



confuses the eye. Two lines not parallel may be combined in one, but then 

 the plane of the combined image will no longer be perpendicular to the eye, 

 but the ends that are nearer each other will appear nearest the eye. Figures 

 that are unequal may be combined, as a triangle with a narrow base with 

 another with a wide one, or even a straight line with the arc of a circle, but 

 the resulting image is always thrown out of plane, and may even be warped. 

 All these must be in a horizontal direction ; two triangles of different heights 

 can never be combined, nor can a horizontal line be combined with the arc 

 of a circle. How is this combination effected ? Brewster believes it is instant- 

 aneous ; "Wheatstone that the eye begins by adjusting one point on one line 

 to the corresponding one of the other, and then another adjoining, and thus 

 gradually forcing an image into the required plane. Brewster maintains 

 that the eye can combine them by the light of an electric spark that lasts only 

 a few millionths of a second. Professor Rogers doubted this, or thought it 

 must at least be a rare case. Sometimes the eye gets fatigued in the opera- 

 tion ; the lines remain open for a time, and at last return to the plane of the 

 paper and remain uncombined. A high degree of approbation was elicited 

 by this paper, which it is difficult to report fairly without copies of the figures 

 and drawings of the apparatus used. 



Professor Holton related an instance of combining two real objects into one 

 image. He was lying in a berth of a steamship, with a Venetian blind within 

 three inches of his face. The slats were horizontal, while his eyes were, of 

 course, one above another. Two slats were combined in one, removed to 

 twice the distance, and doubled in size, so perfectly that the sense of touch 

 was not sufficient to destroy the illusion. He desired also to call the attention 

 of the Section to the importance of raising the stereoscope from a philosophic 

 toy to an important use in descriptive botany and zoology. Means may yet 

 be found for taking photograph pictures of fresh orchid flowers, rare insects, 

 etc., from which colored engravings may be prepared for the stereoscope, 

 giving an idea of forms that could not be otherwise acquired but by models, 

 without a journey to the tropics. 



ON THE BINOCULAR VISION OF SUKFACES OF DIFFERENT 



COLORS. 



The following is an abstract of a paper read at the last meeting of the British 

 Association by Sir David Brewster : Professor Dove had published an account 

 of some beautiful experiments in connection with this subject some years ago. 

 M. Dove showed in his paper that when different colors at the same real 

 distance are regarded by the eye they appear to be at different distances ; 

 this is also the case when a white surface is compared with a black. Now 

 M. Dove argues if a white surface and a black one be stereoscopically com- 

 bined, one of them must be seen through. the other. Taking a figure for the 

 left eye with a white ground, and a second figure of the same object on a 

 black ground for the right eye, when these two figures are combined, a beauti- 

 ful effect is observed : the figure starts into relief, and its sides appear to 

 possess a shining 1 metallic luster. This is the case when the surface of each 



