Mr. Addams on a peculiar Optical Phenomenon. 373 



found that the characters drawn from the combination of the 

 plates, and from the disposition of the ambulacra, form divi- 

 sions more natural and better defined than the characters 

 taken from the position of the mouth and of the anus. 



I shall publish my detailed observations upon this subject 

 in a monograph of the Echi?iodermata 9 accompanied with plates, 

 for which I have already collected the greater part of the ne- 

 cessary materials. 



LI. An Account of a peculiar Optical Phcenomenon seen after 

 having looked at a moving Body, Sfc. By R. Addams, Lee- 

 turer on Chemistry and Natural Philosophy*. 



PJURING a recent tour through the Highlands of Scot- 

 -*-^ land, I visited the celebrated Falls of Foyers on the 

 border of Loch Ness, and there noticed the following phe- 

 nomenon. 



Having steadfastly looked for a few seconds at a particular 

 part of the cascade, admiring the confluence and decussation 

 of the currents forming the liquid drapery of waters, and then 

 suddenly directed my eyes to the left, to observe the vertical 

 face of the sombre age-worn rocks immediately contiguous to 

 the water-fall, I saw the rocky surface as if in motion upwards, 

 and with an apparent velocity equal to that of the descending 

 water, which the moment before had prepared my eyes to 

 behold this singular deception. 



The cascade is through a depth of about 70 feet, and my 

 position, as I stood when I made the observation, was nearly 

 on a level with the centre of the fall, being the lowest of the 

 two situations where visitors obtain a view of this copious and 

 never-failing infusion of peatf gushing over the giant step 

 and whitening as it flows. My attention was engaged on that 

 part of the fall which corresponded with a horizontal plane 

 passing through my eye and the water. The sun was masked 

 by cloud at the time. 



I am not aware of any existing explanation of this class of 

 optical phenomena, and I may be premature in venturing the 

 following. 



I conceive the effect to be owing to an involuntary and un- 

 conscious muscular movement of the eyeball, and thus occa- 

 sioning a displacement of the images on the retina. 



Supposing the eyes to be intently gazing at any point in a 



• Communicated by the Author. 



f The colour is brown from flowing over peat moors. 



