On a Dispersion-Photometer. 117 



As before, the fourth column gives the difference of F and 

 K at 28°-2 C. Taking the mean difference at 28°'2 C. to be 

 947 units, and that at 11° C. to be 45 units, we have 



947-45 KO . ., 

 28'2-H =52 ' 4umts 



as the mean variation-coefficients between 11° C. and 28° C. 

 in units of bridge-wire. Since the coils F and K are approxi- 

 mately ohm coils, this gives as the variation-coefficient of the 

 coil K "0262 per cent. This coil is of platinum-silver wire. 

 These three determinations occupied about an hour and a half, 

 during which time many more readings were taken, all closely 

 agreeing with the above. The actual measurement of the 

 differences requires but a few moments to effect, the principal 

 expenditure of time being that required to bring the coils 

 to the same temperature as the water. 



In conclusion, I may state that this resistance-balance has 

 been constructed in the workshops of the School of Mechanical 

 Engineering at Cambridge, under the direction of Prof. Stuart. 

 Great care was taken in laying on the wire so as to avoid 

 straining it in any way; and the performance of the instrument 

 is consequently very satisfactory. I should also express the 

 fact that this excellent performance is due to the supervising 

 care of Prof. Stuart, who not only supplied several of the 

 details of the construction, but aided, by his valuable sugges- 

 tions generally, during the process of carrying out my rough 

 designs into a practical form. 



Cavendish Laboratory, Cambridge, 

 December 1879. 



XIV. A Dispersion-Photometer. 

 \ By John Perky andW. E. Ayrton*. 



IN measuring what is usually termed the power of a light, 

 it is common to have a screen placed at such a distance 

 from the light that its illumination is equal to that which it or 

 another screen receives from a standard candle. Now, if a 

 standard candle is, say, one foot from a screen, an electric 

 light of, say, 6400 candle-power must be placed at the distance 

 of 80 feet from a screen to give the same illumination. That 

 a great distance like this should be necessary, and in a chamber 

 whose walls are supposed to be blackened, in the laboratories 

 of works where electric lights are usually examined, has placed 



• Communicated by the Physical Society, having been read at the 

 meeting held on December 13, 1879. 



