LIGHT. 117 



generally notice. Thus the purer the spectrum the 

 less yellow is observed. 



If the continuous spectrum be produced with a quartz 

 prism, a little management and adjustment of distance 

 of the screen will cause the two spectra to overlap so 

 that the red of one may be made to coincide with the 

 green, blue, or any desired tint of the other. The 

 same result is obtained by employing two slits at the 

 same time, the distance between which can be adjusted. 

 By this means two spectra are obtained simultaneously, 

 any portions of which may be made to coincide. 



A saturated solution of potassium chromate absorbs 

 all rays more refrangible than the green, while a solu- 

 tion of ammonio, sulphate of copper stops all but the 

 blue and green. These statements may be proved by 

 placing flat cells containing the liquids in front of the 

 slit of the lantern, and on placing one cell in front of the 

 other in the same position, the green light only is trans- 

 mitted. This experiment serves to explain the reason 

 that the mixture of yellow and blue generally results 

 in green, all other rays being absorbed by one or other 

 of the constituents. 



By placing the two cells in front of separate lanterns 

 and throwing disks of light upon the screen, a beauti- 

 fully pure white is produced when the blue and yellow 

 overlap. I employ one lantern only for this exper- 

 iment, using two focussing lenses side by side to pro- 

 duce the overlapping circles of light. I also employ 

 a cell with three compartments, containing solutions of 

 analine, ammonio, sulphate of copper, and a mixture 

 of potassium chromate with the last solution, and pro- 

 jecting images on the screen by means of three lenses 

 fitted on the same stand but capable of separate ad- 

 justment. 



