144 Mr. Dyke on the Determination of the Mean Spherical 



Readings were obtained with and without the screen in 

 position, the absorbing power being taken as the ratio of the 

 illumination on the photometer without the screen to the 

 illumination with the screen. 



Observations were made on the Nernst lamp from 20 per 

 cent, below its marked volts until the automatic heater came 

 into action, and on the Focus lamp from 2 per cent, above to 

 10 per cent, below its marked volts. 



The results of these experiments are shown in the accom- 

 panying curve (PL III. fig. 6), in which absorbing power is 

 plotted against watts per candle. 



The curve shows the general result that the absorption 

 increases with the watts per candle, whence it follows that 

 the red rays suffer a greater absorption than the blue. 



It was at once seen from these results that any such screen 

 could not possibly be used for sources of different spectral 

 character, and hence the somewhat bulky apparatus of fig. 3 

 was necessitated. 



Results. 



Incandescent Lamps. 



In all, nine types of incandescent lamps were tested ; the 

 total number of lamps under observation being twenty-five. 



The horizontal candle-power was investigated as indicated 

 above, the average number of positions photometered being 

 26 for 16 C.P. lamps and 84 for 32 O.P. lamps ; the greater 

 number in the case of the 32 C.P. lamps being necessitated 

 by the greater distance between lamp and photometer, and 

 the consequent diminished sharpness in the gradations of 

 illumination of the photometer as the lamp was turned round 

 by hand. 



The following is a table of the different types of lamps 

 investigated : — 



1. Ediswan Flat-Loop Filament. 50-16-B 1-12. 



2. Ediswan Double-Loop Filament. 100-16-A *64. 



3. Edison Double-Loop Anchored Filament. 110-V-16 



C.P.-E. 



4. Ediswan Triple-Loop Doubly Anchored Filament. 



110-16-A. 



5. Ediswan Triple-Loop Doubly Anchored Filament. 



210-16-B. 



6. Royal Ediswan Double-Loop Filament. 110-16-A '58. 



7. Ediswan Double-Loop Filament. 110-8-A *29. 



8. Ediswan Quadruple-Loop Doubly Supported Filament. 



110-32-B. 



9. Premier Double-Loop Filament. 32-110-D. 0. 



