Simplified Dispersion-Photometer. 49 



the distance of the centre of the lens from the screen, and c that 

 of the candle when the shadows show equal illumination, then, 

 if L is the strength of the examined source of light in standard 

 candles, 



^5=*{i + 4 +D i,)}. 



or 



For our own use we prefer to employ the formula ; but as all 

 the common instruments which have hitherto been manufac- 

 tured have lenses whose focal length is 4 inches, we have pre- 

 pared a table, a copy of which is sent out along with each 

 instrument, in which the value of L is given for various 

 values of D, d, and c. Using this table, it is necessary to 

 have the lamp at either 60, 120, or 300 inches from the screen; 

 the candle is either at 10, 14" 14, or 20 inches from the screen; 

 and the table is made out for every half inch of the lens-scale. 

 But inasmuch as we find that the improved arrangement of 

 the mirror already referred to constitutes perhaps the most 

 useful part of the instrument, and as the use of this improve- 

 ment involves many alterations of D, the manufacturer pro- 

 poses in future not to furnish any table of values of L unless 

 specially asked for. 



H is the plane silvered-glass mirror which makes the angle 

 of 45° with the axis of the lens, and with the axis about which 

 the mirror itself is free to revolve. As already explained, a 

 ray of light reflected from the mirror and passing through the 

 centre of the lens must, for any position of the mirror, have 

 an angle of incidence of 45°, and so must experience the same 

 amount of absorption, from whatever direction it may have 

 come to the mirror. Further, .this angle being 45°, a fixed 

 pointer marks on the graduated circle Gr, which moves with 

 the mirror, the angle which any ray we may be examining 

 makes with the horizontal. 



In this instrument we find that from 30 to 34 per cent, of 

 the incident light at 45° is absorbed, whether this light is of 

 ruby-red or signal-green colour ; so that we have the easy 

 practical rule for all cases— add one half to the measured 

 intensity of light reflected. 



We need not here refer to the fact that, when investigating 

 the efficiency of an electric lamp, we always measure the 

 horse-power given electrically to the lamp simultaneously 

 with the photometric measurement. 



The lamp is suspended in such a way that it can readily be 

 Phil. Mag. S. 5. Vol. 14. No. 85. July 1882. E * 



