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LXXV. On the Rev. J. G. MacVicar's Experiment on Vision. 

 By J. T. Graves, Esq., A.M.* 



nPHE ordinary laws of reflection and perspective are suffi- 

 -^ cient to account for the radiated appearance described by 

 the Rev. Mr. Mac Vicar {ante, p. 234,) without the hypothesis 

 of any hitherto unknown symmetrizing power in the eye. 



No radiation is observed when the particles are spread at 

 whatever distance over a non-reflecting surface, or are placed 

 in immediate contact with a reflecting one. Now if a reflecting 

 surface in the required position of contiguous parallelism to 

 the plane of the dust contributes to the success of the experi- 

 ment, it is surely a priori likely that it does so rather by the 

 addition it makes to the objects of vision than by any induced 

 alteration in the mechanism of the eye, affecting the mode in 

 which the former objects, if seen alone, would be viewed. Ac- 

 cordingly we are led to consider the actual nature of the pic- 

 ture before us when the phaenomenon is observed. 



When we survey with one eye dust spread over a common 

 looking-glass, we see not only the particles themselves but 

 their images reflected by the mercury (wherever those images 

 are not intercepted by the position of other particles on the 

 glass), and both classes of objects, from their general similarity 

 of appearance, are referred to the same plane. Upon further 

 examination we shall perceive that every reflected image when 

 referred to the external surface of the looking-glass, appears 

 in a direct line between its prototype and the point of that 

 external surface nearest the centre of the eye. In making this 

 observation, we may use with advantage small shots or seeds 

 instead of flour or other finely-powdered substances, and we 

 may put out of consideration the fainter and scarcely percep- 

 tible images formed by reflection at the external surface, as 

 they do not materially alter the general picture. Now the 

 appearances above described might have been predicted as 

 the necessary results of known principles. 



Within the ordinary limits of the sphere of sight, it is an 

 observed approximate law of vision, whether direct or in- 

 direct, that when a pencil of rays diverging from any point 

 enters the pupil, an image is seen in the direction of the line 

 drawn from that focal point through the centre of the eye. 

 Again, it is an observed law of light that the angles of inci- 

 dence and reflection are equal and in the same plane. From 

 the latter law it follows by simple geometrical reasoning, that 

 all the rays proceeding from any point and reflected by a 



* Communicated by the Author. 



