Illumination and the Measurement of Light. 



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inside a cylinder or box (B) blackened in the interior, over the end of 

 which is stretched a diaphragm of drawing paper (P) ; the latter has 

 a round grease spot in the centre about the size of a shilling, as in 

 Bunsen's photometer. At about 12 inches distant from the end of 

 the tube a screen (D) of white drawing paper is fixed. The dia- 



Fig. 2. 



phragm (P) is so screened by a sliding cover that no light falls upon 

 it but that which is reflected from the drawing paper (D). At the 

 end of the tube where the glow lamp L is set is an eyepiece, through 

 which the illuminated surface of the diaphragm at the end of the 

 tube can be observed. Beneath the base of the instrument is fixed a 

 secondary battery (e, fig. 2) which enables us to transmit a current of 

 any desired strength through the glow lamp (L). A rheostat (B) 

 is fixed in circuit with this battery, and enables the current to be 



