refractions, and of being capable of decomposition by absorption alone.” This analysis of light has been called in question by Professor Challis, of Cambridge, who does not admit of any resolution by absorbing media different from that by the prism, though he admits that a mixture of blue and yellow solar light produces green. Professor Stokes, on the contrary, does not allow that a mixture of blue and yellow solar light produces green, although that mixture produces green when the light is from other sources, for he found the gradation from sunlight to pass from yellow through diluted yellow, white, diluted blue to blue; so he does not admit of Sir David Brewster’s analysis of the spectrum; however, there appears to be still a doubt as to the real character of the phenomena presented by certain absorbing substances. In addition to the seven colours of the Newtonian spectrum, Sir John Herschel has discovered a set of very dark red rays beyond the red extremity of the spectrum which can only be seen when the eye is defended from the glare of the other colours by a dark blue cobalt glass. He has also found that beyond the extreme violet there are visible rays of a lavender gray colour, which may be seen by throwing the spectrum on a sheet of paper moistened by the carbonate of soda. The illuminating power of the different rays of the spectrum varies with the colour. The most intense light is in the mean yellow ray, or, according to M. Fraunhofer, at the boundary of the orange and yellow.