336 



in the Transactions of this Society, and ascribing tlieni to succes- 

 sive subsidences of a fresh-water lake, is still the only view re- 

 concilable with the facts. 



James Spittal, M.D., Fellow of the Royal College of Phy- 

 sicians of Edinburgh, was duly elected an Ordinary Fellow. 



2. On the Visibility of rapidly revolving Lights, made in re- 

 ference to the Improvement of Light-Houses. By Alan 

 Stevenson, LL.B , Civil Engineer. 



These experiments consisted in a comparison of the visibility 

 of lights from lenses when at rest, and when revolving with such 

 rapidity as to produce an apparently continuous impression on 

 the sense of sight. They were undertaken at the suggestion of 

 Captain Basil Hall, who had himself in the spring of last year 

 made some trials of a similar kind, in the expectation that the 

 eye would be so stimulated by the bright flashes, that not only 

 the almost imperceptible intervals of darkness would have no 

 effect in impairing the visibility of the rapidly recurring flashes, 

 but likewise the eye would actually be stimulated by the contrast 

 of light and darkness, in such a manner that the effect of the 

 rapid series would be greater than that of the same quantity of 

 light equally distributed over the whole horizon by the refracting 

 zones at present used in fixed lights, which only refract the light 

 in the vertical direction, without interfering with its natural ho- 

 rizontal divergence. Mr Stevenson shewed that this expectation 

 was at variance with what would be predicted from a considera- 

 tion of the laws of the physical distribution of the light; and the 

 experiments proved that the visibility of the rapidly revolving 

 series was greatly inferior, not only to that of the lens at rest, but 

 also to that of the light equally distributed by the refracting zones. 

 From the results of the experiments, the author drew the follow- 

 ing general conclusions : — 



1. That continuity of impression in the sense of sight is scarcely 

 obtained by producing ten flashes in a second of time ; and that 

 the visibility of the light decreases in a most remarkable degree 

 with the velocity of the series. 



2. That this decrease of visibility, although partly owing to a 

 loss of intensity, is chiefly caused by deficiency of volume in the 

 visual object, which at the most rapid velocity became so small 



