231 



the author has succeeded in rendering visible, by making the wheel 

 revolve on an axle of less than its own diameter ; and the appear- 

 ances being in this, as well as in the foregoing case, perfectly con- 

 sonant to his theory, he considers the explanation given as quite 

 satisfactory. 



Dr. Roget concludes by suggesting the possibility of measuring 

 the duration of the impression of light on the retina by observing 

 the apparent velocity of the visible portion of the spokes. 



On a new Photometer, with its application to determine the relative In- 

 tensities of Artificial Light, #c. By William Ritchie, A.M., Rector 

 of the Academy at Tain. Communicated by the President. Read 

 December 16, 1824. [Phil. Trans. 1825,^. 141.] 



Mr. Ritchie, after a brief exposition of the theoretical views which 

 led him to the construction of his photometer, lays down the fol- 

 lowing as the principles on which it depends : 



1 . That radiant heat does not permeate glass. 



2. That light is capable of combining with substances which stop 

 it, and expanding them as heat does. 



3. That the intensity of light is in the inverse ratio of the squares 

 of the distance. 



The photometer, which he then proceeds to describe, consists of 

 two tin-plate cylinders, broad and shallow, each of which is closed at 

 the one end with tin plate, and at the other with a disk of the thick- 

 est plate glass, both made air tight. Each of these cylinders or 

 chambers contains in the middle a diaphragm of black paper, with 

 its black side towards the glass, for the purpose (as he expresses it,) 

 of absorbing the light which penetrates the glass, and instantly con- 

 verting it into heat. The chambers are then fixed back to back at a 

 little distance from each other, and connected by a bent tube in the 

 form of the letter U, containing a small quantity of a coloured liquid. 



This instrument is exposed with its glass faces opposite to two 

 lights to be compared, and their equality is judged of by the liquid 

 in the stem remaining stationary. 



Its sensibility is described by the author as such, that a single 

 candle placed 10, 20, or 30 feet from it visibly affects it; while a mass 

 of heated iron affording twenty times the heat has no influence. 



The author proposes his photometer as peculiarly well adapted to 

 the measure of the quantity of light given out by gas lights. The 

 solar light he describes as powerful enough to drive the liquid in the 

 stem through 20 or 30 feet of tube. He states himself to have an 

 instrument of this kind now making, with which he hopes to render 

 sensible the effect of the moon's rays. Finally, he explains the dif- 

 ference between his instrument and that of Professor Leslie to consist 

 in this, that in the latter the difference of temperature between the 

 two balls is the quantity measured ; in the former the perfect equality 

 of their temperatures is the essential condition. 



