AN observer who for the first time views the light of the sun through a prism cannot fail to express his wonder and delight at the gorgeous display of colours into which the. white light is separated; and if the observation is made under the same conditions as in the celebrated experiment of Newton, 1666, there is, in truth, nothing else which he could observe. You will remember that he allowed a beam of sunlight to stream through a round opening in a shutter of his window, falling on a glass prism, which bent the sun-rays by different amounts depending on their colour, thus spreading out the white round sunlit spot on the opposite wall into a coloured band—the spectrum-which he rather arbitrarily divided into seven colours-red, orange, yellow, green, blue, indigo, and violet. (If the division were made to-day I doubt if indigo would be included.) There is, in fact, no definite demarcation between these, and they shade insensibly into each other, and if the solar spectrum were always produced under these conditions we should say it was continuous; indeed, if it were not the sun, but an argand burner or an incandescent lamp, which served as source, it would really be so.