for Colour- Combinations. 83 



ing half of the aperture in the screen is seen illuminated with 

 a colour which will depend on the position of the incandescent 

 wire, and which will be pure and uniform if the wire be not 

 too thick, the slit in the eyepiece too wide, or the strip of the 

 prism exposed too large. The colours are found in practice to 

 be perfectly uniform. 



The prisms and lenses are supported at the level of the in- 

 candescent wires, on a horizontal floor (Gr), which divides the 

 box two inches from the top into two compartments, and 

 which is cut away so as to leave a space for the tops of the 

 frames carrying the wires to enter the upper compartment. 

 These frames (W), which are six in number, three on each 

 side, consist of brass wires held on small ebonite blocks (I), 

 which slide along a steel rod (H) supported on a ledge inside 

 the box. The frames can be moved along the length of the 

 spectrum from outside by means of openings (A) cut in the 

 sides of the box, into which the hands can be introduced. 

 These openings are below the level of the horizontal floor, so 

 that the upper compartment, which is everywhere blackened 

 inside, is quite impervious to external light, unless the lids in 

 the cover of the box, to be presently described, are opened. 



The frames are connected by flexible wires to a series of 

 resistances arranged in the lower compartment of the box; and 

 by means of six dials (R), on the sides of the box, each of the 

 six circuits can be altered in resistance at will, so that the 

 incandescent wires can be rapidly adjusted, whether in position 

 or in intensity, by the observer without his having to remove 

 his eye from the eyepiece. Six pairs of binding-screws (T) 

 at the back of the box connect the six circuits with a Grove 

 battery. By this means, when the current is passing, the 

 aperture in the screen is seen divided into two coloured strips 

 in close juxtaposition ; one colour being that due to the wires 

 and prisms on the right hand, the other to those on the left. 

 The colours can thus be readily compared and adjusted to 

 exact identity. 



It remains to describe the means by which external colours 

 can be introduced for comparison with those produced by the 

 prisms. 



The cover of the box contains three small hinged lids. Two 

 of these (0) are over the luminous wires, and serve to adjust 

 or replace the wires in case of accidental fusion. The third 

 lid (D) opens over the space just in front of the screen. Here 

 is placed a small strip of microscope cover-glass (M), inclined 

 at about 60° to the horizontal, and so arranged that, as seen 

 from the eyepiece, it exactly covers that portion of the aper- 

 ture in the screen which is illuminated by the light from the 



G2 



