On the Rev. J. G. Mac Vicar's Experiment on Vision, 371 



plane mirror diverge after reflection from an opposite corre- 

 sponding point equidistant from the mirror; and it also fol- 

 lows that the line joining the conjugate points from which 

 direct and reflected pencils diverge is perpendicular to the plane 

 of the mirror. Hence if we have a perspective plane pa- 

 rallel to the plane of the mirror, and carrying all the ob- 

 jects to be reflected, the line of direction in which, by the pre- 

 ceding law of vision, any reflected image is seen will cut the 

 line which joins the object and the point of the perspective 

 plane nearest the centre of the eye, in the ratio which twice 

 the distance of the perspective plane from the mirror bears to 

 the distance of the same plane from the centre of the eye. 

 While the head remains unmoved, the rolling motion of the 

 eye does not affect the position of its centre, and consequently 

 the apparent positions of the reflected images as well as those 

 of their direct objects remain unaltered. 



The relative position of the particles and their images to the 

 point of the perspective plane nearest the centre of the eye 

 will account for the radiated appearance. The interval be- 

 tween each particle and each image produces a short line 

 pointing in a given direction. The multitude of such pointers 

 and the multitude of instances in which separate pointers com- 

 bine to form a prolonged continuous line radiating to a given 

 centre, actually produce in the spectrum a predominant sym- 

 metry, which the mind, without any previous peculiar adjust- 

 ment of the eye, can scarcely fail to notice. 



We may form a similar radiating group of objects of vision, 

 which shall not have the disadvantage of being altered as to 

 half its components by every motion of the centre of the eye, 

 if we first dot a sheet of paper indiscriminately, and then 

 mark a second system of dots so related to the former that the 

 interval between each pair of conjugate dots may tend in di- 

 rection to an assumed centre and be proportional in length to 

 its mean distance from that centre. Here it is evident on in- 

 spection that no extraordinary symmetrizing power is required 

 to perceive the radiation. We may observe, however, that if 

 we mark the second system only to a certain distance, around 

 the assumed centre, a principle of continuity will incline us 

 to trace the resulting radiation beyond that distance, by draw- 

 ing our chief attention to the exterior points in the original 

 system in so far as they continue the discovered symmetry. 

 Again, if we mark the first system of dots in blue, and the 

 second in red, the radiation resulting from the combined po* 

 sition of the systems will fix our attention on the parts of each 

 system in so fer as they contribute to the joint effect, so that 

 when the two systems are simultaneously viewed as separate 



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