84 On a new Apparatus for Colour- Combinations. 



left-hand train of prisms, this light being thus seen through 

 the glass slightly diminished in intensity. On the underside 

 of the lid is a small silvered mirror (N); and by raising the 

 lid to the proper height, the colour of any object laid on the 

 top of the box is seen by double reflection in the eyepiece, 

 exactly coinciding with the left half of the aperture in the 

 screen. This colour may be either combined with that from 

 the left-hand train of prisms, or compared with that from the 

 right-hand train. For white light I use the light from a 

 window, which may be reduced in intensity to any desired 

 amount by passing it through one or more pieces of roughened 

 glass, held in a frame placed on the top of the box. Coloured 

 glasses, or cells of coloured fluids, can similarly be placed in 

 the frame, so that almost every possible combination or com- 

 parison of colours may be made. Another frame, which 

 may be placed inside the box between the screen and the 

 eyepiece, serves to show the action of different absorbing 

 media on colours identical in appearance but of different 

 composition. 



The colours produced are uniform, very constant, and fairly 

 bright. Those from the blue and violet parts of the spectrum 

 are necessarily more deficient in luminosity. This is partially 

 compensated by using in the less luminous parts of the spec- 

 trum ribbons of platinum, produced by rolling out the wires, 

 with their flat surfaces turned towards the lenses. This is 

 equivalent to increasing the width of the slit in ordinary 

 arrangements. Should the wires become fused through in- 

 cautiously using too strong a current, they can be very readily 

 replaced by means of the screws (K). 



For the current, three Grove's cells are needed for each of 

 the ribbons and two for each of the wires. But since the 

 frames carrying the wires can be brought together till the 

 wires are nearly in contact, and since the wires themselves 

 offer extremely narrow luminous surfaces, a very long spec- 

 trum is not needed ; and by using more powerful lenses and 

 bringing the sliding frames closer to them, a greater intensity 

 of light would result; while the size of the box might thus be 

 reduced to one half or even less of its present amount, the 

 eyepiece being kept at a sufficient distance by the use of a 

 longer tube, which might be made removable. One Grove 

 cell would then suffice for each wire, or at most two when 

 experimenting On the violet rays. 



Instead of the step-by-step dial-resistances, some arrange- 

 ment giving, like the rheostat, a gradual adjustment of the 

 intensity would be perhaps preferable. 



Scales can be attached in front of the sliding frames to 



