26 Mr. Robert Sabine on a 



giving the values of — ~, which is determined for each position 



of the wedge by obtaining a balance with a constant light at 

 different distances, and for each of the diaphragms controlling 

 the illumination of the comparison-field. In using the photo- 

 meter in practice, it is therefore only necessary to obtain a 

 balance and to read off the indication of the scale. A refer- 

 ence to the table then gives the value which, multiplied into 

 the square of the distance in metres, gives the candle-power of 

 the measured light, or, if the distance be exactly 1 metre, 

 gives the candle-power direct. 



It happens sometimes that the degree of illumination to be 

 ascertained is independent of distance, as in the case of day- 

 light ; in this case the diaphragm and wedge are adjusted 

 until a balance is obtained. The value given by the table 

 then represents the number of standard candles which at 

 1 metre distance would illuminate an object to an equal 

 degree. 



In measuring the illuminating-powers of lights the colours 

 of the lights have sometimes to be taken into account. The 

 way I prefer to do this is to assume the ingredient of orange 

 light contained in daylight as unit for comparing the relative 

 ingredient of orange light contained in the artificial light 

 which is measured. By orange light I mean that ingredient 

 of light which passes freely through orange glass, and which 

 in fact embraces nearly all that portion of the visible spectrum 

 between the lines B and F — that is to say, the red, orange, 

 yellow, and a small part of the green rays ; whilst with a 

 moderate illumination, such as is used in photometric compa- 

 risons, all the more refrangible and less luminous rays from 

 the line F upwards are quenched. 



For convenience of observing, the photometer is furnished 

 with a small disk centred inside the eyepiece and containing 

 small panes of white and orange glass*, either of which can 

 be instantly interposed when required. 



A balance of illuminating effect is first estimated with the 

 white pane irrespective of colour; then the disk is turned so 

 as to interpose the orange pane, and the wedge readjusted 



accordingly. If with the orange glass the reading is < l £ ei \ 

 than with the white (that is, if j more f light nas *° ^ e a ^~ 

 mitted through the wedge in order to maintain the balance), 



* The disk contains also red and green glasses, and may be furnished 

 with any other colours for which the observer has a fancy. 



