Luminous Intensity of Light. 



367 



piece (shown separate at D C) slipping into the end B. The sloping sides, 

 DB, BC, are covered with a white reflecting surface (white paper or finely 



Fig. 2. 



ground porcelain), so that when D C is pushed into the end B, one white 

 surface D B may be illuminated (as in fig. 2) by the candle, and the other 

 surface B C by the lamp. If the eyepiece A is removed, the observer, 

 looking down the tube G B, will see at the end a luminous white disk 

 divided vertically into two parts., one half being illuminated by the candle 

 E, and the other half by the lamp F. By moving the candle E, for in- 

 stance, along the scale, the illumination of the half D B can be varied at 

 will, the illumination of the other half remaining stationary. 



The eyepiece A (shown enlarged at fig. 3) will be understood by refer- 

 ence to fig. 1, the same letters representing similar 

 parts. At L is a lens to collect the rays from DB C x 

 (fig. 2), and throw the image into the proper part 

 of the tube. At M is another lens so adjusted as 

 to give a sharp image of the two disks into which 

 I is divided by the prism K. The part N is an 

 adaptation of Arago's polarimeter ; it consists of a 

 series of thin plates of glass capable of moving 

 round the axis of the tube, and furnished with a s 

 pointer and graduated arc (shown at A G, fig. 2). 

 By means of this pile it is possible to partially 1 

 polarize the rays coming from the illuminated disks 

 in one or the other direction, and thus bring to the 

 neutral state the partially polarized beam c e?(fig. 1), 

 so as to get the images g, r free from colour. It is 

 so adjusted that when at the zero-point it produces an 31 

 equal effect on both disks. 



The action of the instrument is as follows. The 

 standard lamp being placed on one of the supporting pillars which slide along 

 the graduated stem (fig. 2), it is adjusted to the proper height, and moved 



