188 A. Pedler — On the use oftJie 'Radiometer as a Photometer. [August, 



and the experiments which are here described were performed with one 

 of the second class of radiometers, which had been forwarded to me 

 from England. In a jDaper " on the Mechanical Action of Light" by 

 Mr. Crookes,* a few photometrical experiments with this instrument 

 are given, and from them it is concluded that the radiometer is a perfect 

 photometer. The author says " By this means Photometry becomes much 

 simplified, flames the most diverse may readily be compared between them- 

 selves or with other sources of light ; a standard candle can now be defiaied 

 as one which at ce inches off causes the radiometer to perform y revolutions 

 per minute, the values of a? and y having previously been determined by 

 comparison with some ascertained standard ; and the statement that a gas 

 flame is equal to so many candles may with more accuracy be replaced by 

 saying that it produces so many revolutions." This conclusion being of 

 great practical importance, and as the expei-iments on which it was based 

 were very few in number, it appeared to be advisable that they shoiild be, if 

 possible, confirmed by a more extended series of observations. For this 

 purpose during the past six weeks, I have made a continuous series of 

 measurements with this instrument, which do not however enable me to 

 speak with great confidence in the radiometer as a photometer. 



The mechanical effect produced in a radiometer is admittedly the 

 product of the two forces, light and heat, and as it is well known that the 

 illuminating power of a gas jet or candle flame depends very essentially 

 upon its temperature, I thought at first that it would be better when testing 

 the radiometer photometrically to employ the total radiation from the 

 flames. 



My first experiments consisted in observing the radiometer, which was 

 placed at a fixed distance from a gas flame, at the same time that I was 

 testing the gas flame by the old photometrical method of Bunsen. For this 

 purpose the radiometer was placed inside the photometer, in which it has 

 been kept during the whole of the experiments ; this photometer is entirely 

 lined with black velvet, so that we have only to deal with the radiation 

 from the hght itself, and the phenomena are not complicated by any radia- 

 tion from extraneous sources, as would be the case if the experiments were 

 performed in an open room. The distance of the radiometer from the gas 

 jet in these preliminary experiments was 27"2 inches ; the gas-jet a standard 

 argand one, burning 5 feet of gas per hour, and the observations of the 

 radiometer arc here given in quarter revolutions, that is to say, the number 

 of arms of the vane which pass a given spot in a given time. 



* Quarterly Journal of Science, July, 1875. 



