358 SIR DAVID BREWSTER ON THE LAW OF VISIBLE POSITION 



is made to pass into A, I have never succeeded in making b pass into B. When- 

 ever there is an appearance of this, either turn round the paper, or the head, so 

 as to separate the lines as in fig. 6, and it will be invariably seen that if a springs 

 out of A, b will spring out of a point between A and B. The apparent coincidence, 

 therefore, of AB with a b, fig. 6, when it is seen, arises from the disappearance of 

 one or other of the extremities of the two lines. 



But Mr WHEATSTONE has described another very interesting experiment, of 

 the same character as that which we have been examining, and he regards it as 

 " proving that similar pictures, falling on corresponding points of the two retinae, 

 may appear double and in different places." Draw a strong vertical line, AB, 

 fig. 7. and another CD inclined ]some degrees to it, and also a faint line m n paral- 

 lel to AB, and cutting CD at its centre S, then, according to Mr WHEATSTONE, 

 the two strong lines AB, CD, when seen with different eyes in the stereoscope, or 

 brought together by looking at a nearer object, " will coincide, and the resultant 

 perspective line (CD) will appear to occupy the same place as before ; but the 

 faint line (m n) which now falls on a line of the left retina, which corresponds 

 with the line of the right retina, on which one of the coinciding strong lines, viz., 

 the vertical one (AB) falls, appears in a different place." In repeating this ex- 

 periment, I have occasionally observed an apparent coincidence similar to that 

 which is described in the preceding passage ; but after numerous and varied obser- 

 vations, made with lines coloured and uncoloured, opaque and transparent, similar 

 and dissimilar both in strength and form, I have no hesitation in affirming that 

 the phenomenon described by Mr WHEATSTONE is an illusion, arising from the 

 actual disappearance of one or more parts, or even of the whole of one of the 

 lines, and from the difficulty of observing the separation or superposition of images 

 in the circumstances under which the experiment is made. 



The following are a few of the variations of the experiment which I have 

 found the best calculated to exhibit the real place of the combined images. 



1. In Mr WIIEATSTONE'S form of the lines shewn in fig. 7, the strong line A B 

 assumes more readily the appearance of uniting with the similarly strong line C D ; 

 but if m n is a strong line and CD a weak one (fig. 11), or an interrupted one, AB 

 will unite with m n, and not even apparently with CD. In like manner, if AB be a 

 weakline, it will unite with the weak line m n rather than with CD. (See fig. 12.) 



Now, the apparent coalescence of similar lines arises from the fact, that when 

 corresponding, or nearly corresponding, parts of the retinas are impressed with 

 similar images, one of the two more readily vanishes, independent of its liability 

 to vanish from its being out of the axis of vision. Whenever two images inter- 

 fere with one another so as to impede vision, one of them disappears or rather, 

 is not taken cognisance of by the eye. Hence it is, that many sportsmen shoot 



