Philosophical Transactions, 299 



principal. effect of curvature is observed to come on suddenly, but 

 the degree of it is independent of the velocity of the wheel, each 

 image appearing motionless during the moment that it is viewed. 

 The deception is improved by every circumstance which tends to 

 abstract attention from the bars, and fix it on the wheel. 



The number of curved images was found to depend upon the 

 ratio of the angles subtended at the eye by the intervals between 

 the bars, and those between the extremities of the spokes, being 

 greater as this ratio was less. The combination of a progressive 

 with a rotatory motion Avas also found essential to the phenomena. 



From these and other circumstances, Dr. Roget refers this de- 

 ception to the principle that an impression made by a pencil of 

 rays on the retina, will, if sufficiently vivid, remain for a certain 

 time after the cause has been withdrawn. He enters into a 

 lengthy application of this principle to the case before us, and 

 shows that the apparent form of each spoke will be a curve 

 formed by the continual intersection of the revolving and ad- 

 vancing radius with the immoveable interval between the bars 

 referred to the plane of the wheel. The general form of these 

 curves he refers to the class of Quadratrices, and the most re- 

 markable among them is that discovered by Dinostrates, and 

 known by his name. 



vi. On a nexo Photometer., with its Application to determine the 

 relative Intensities of artijicial Light, ^c. By William Ritchie, 

 A. M., Rector of the Academy at Tain. Communicated by the 

 President. 



This new photometer consists of two cylinders of planished tin 

 plate from 2 to 10 or 12 inches in diameter, and from a quar- 

 ter of an inch to an inch deep. One end of each cylinder is en- 

 closed by a circular plate of the same metal soldered completely 

 air-tight, the other ends being shut up by circular plates of the 

 finest and thickest plate-glass, made perfectly air-tight. Half 

 way between the plates of glass and the ends of the cylinders, 

 there is a circular piece of black bibulous paper, for the purpose 

 of absorbing the light which permeates the glass, and converting 

 it into heat. 



The two cylinders are connected by small pieces of thermome- 

 ter-tube which keep them steady with their faces parallel to 

 each other, but turned in opposite directions, and also serve to 

 make the insulation as complete as possible. The chambers are 

 then connected by a small bent tube in the form of the letter U, 

 having small bulbs near its upper extremities, and containing a 

 little sulphuric acid, tinged with carmine. The instrument is 

 supported upon a pedestal, having a vertical opening through the 

 stem to allow the glass-tube to pass along it, and thus secure 

 it from accidents. A small scale divided into any number of equal 

 parts, is attached to each branch of the tube, 



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