IN OPTICS. 71 



had we been created unembodied spirits, but 

 with the same faculties of perception as we 

 enjoy at present, we could no more have judged 

 one line to be perpendicular, and another to 

 be parallel to the horizon, than we can at pre- 

 sent determine, without some external aid, 

 which is the eastern, and which the western 

 point of the heavens. I shall now draw from 

 these principles, the explanation of a fact, 

 which was first mentioned by one of the most 

 ingenious authors that have written upon vision, 

 but left by him still to be justly accounted for. 

 " I have frequently" (says Mr. Melvill)* 

 <c observed, when at sea, that, though I pressed 

 my body and head firmly to a corner of the 

 cabin, so as to be at rest in respect to every 

 object about me, the different irregular mo- 

 tions of the ship, in rolling and pitching, were 

 still discernible by sight. How is this fact 

 to be reconciled to optical principles? Shall 

 we conclude that the eye, by the sudden 

 motions of the vessel, is rolled out of its due 

 position? Ov, if it retains a fixed situation 

 in the head, is the perception of the ship's 

 motion, owing to a vertigo in the brain, a de- 

 ception of the imagination, or to what other 

 cause ?" 



* Edinburgh Physical Essays, vol. ii. p. 8O. 



