372 On the. Rev. J. G. Mac Vicar's Ejcpetiment oti Vision, 



wholes, they will each appear to radiate. If this were re- 

 peated often enough, we might perhaps afterwards be able to 

 trace a radiation in either system unbacked by the guidance 

 of the other, for the eye is predisposed to recur to that atti- 

 tude in which while previously looking at an object symmetry 

 has been perceived. 



The eye has parts severally capable within certain limits 

 of distinct adjustments voluntary or instinctive. From natural 

 conformation, acquired habit, or the present immediate opera- 

 tion of metaphysical causes, the eye seeing simultaneously the 

 whole of a given field, may probably possess a superior facility 

 of severally adjusting its mechanism by parts corresponding 

 to parts of the field in some manners rather than in others, 

 and may also be predominantly inclined to alter its sphere of 

 vision in particular directions. Hence in a group of objects 

 in which different laws of arrangement are discoverable, one 

 attitude of the eye may be the best adapted for the discovery 

 of one law and another for that of a different one. For example, 

 let a sheet of paper be dotted in such a manner that each sur- 

 rounded dot may be the centre of a regular hexagon, of which 

 the surrounding dots mark out the apices ; then the rows, in 

 which the mind will lioithout effort arrange the dots, will vary 

 with the direction of the eye, and, with effort, the grouping 

 may be altered without altering the field of vision. A similar 

 observation may be made in looking at large piles of cannon 

 balls. Not the imagination merely but the plastic eye of 

 Phidias is at work while he is delineating the Venus in the 

 yet uncarved marble before him. That visual prejudice which 

 consists in a tendency to adjust the respective parts of the eye 

 in particular modes, not naturally suggested by the general 

 impression resulting from the objects presented to those parts, 

 is a defect exemplified in persons who see ghosts in the win- 

 dow-curtains and faces in the burning embers. 



Though the interesting experiment which Mr. Mac Vicar 

 has introduced to the public in the pages of this Magazine, 

 does not seem adapted to illustrate the changes that take place 

 in the eye, corresponding to the changes that take place in the 

 grouping and symmetry seen in the subjective spectrum of 

 a given object, it may be of use for a purpose of scarcely in- 

 ferior curiousness and importance; for if we compare what 

 we see in surveying the powdered looking-glass with both eyes, 

 with the phaenomena which we should expect to result from 

 the separate and different spectra due to each eye singly, we 

 may perhaps be enabled to throw light on some obscure pro- 

 cesses in the physiological mechanism of binocular vision. 

 This, however, is an inquiry deserving consideration by itself* 



