An explanation of this curious defect will be worth listening to, the more so as one of our most eminent philosophers, Sir John Herschel, has recently made a few remarks on the subject, directing attention at the same time to other little known but not unimportant phenomena of colour, which bear upon and help to explain it. It is known that white light consists of the admixture of coloured rays in certain proportions, and that the beautiful prismatic colours seen in the rainbow are produced by the different degree in which the various rays of colour are bent when passing from one transparent substance into another of different density. Thus, when a small group of colour-rays, forming a single pencil or beam of white sunlight, passes into and through the atmosphere during a partial shower, and falls on a drop of rain, it is first bent aside on entering the drop, then reflected from the inside surface at the back of the drop, and ultimately emerges in an opposite direction to its original one. During these changes, however, although all the colour-rays forming the white pencil have been bent, each has been bent at a different angle—the red most, and the blue least. When therefore they come out of the drop, the red rays are quite separated from the blue, and when the beam reaches its destination, the various colours enter the eye separately, forming a line of variously coloured light, the upper part red and the lower part blue, instead of a mere point of white light, as the ray would have appeared if seen before it entered the drop. The eye naturally refers each part of the ray to the place from whence it appears to come, and thus, with a number of drops falling and the sun not obscured, a rainbow is seen, which represents part of a number of concentric circular lines of colour, the outermost of which is red, the innermost violet, and the intermediate ones we respectively name orange, yellow, green, blue, and indigo. It has also been found by careful experiment, that these are not all pure colours, most of them being mixtures of some few that are really primitive and pure, and necessarily belong to solar light. It is these mixed in due proportion which make up ordinary white light, which is the only kind seen when the sun’s rays have not undergone this sort of decomposition or separation into elements. The actual primitive colours are generally supposed to be red, yellow, and blue, and much theoretical as well as practical discussion has arisen as to how these require to be mixed, what proportion they bear to each other in their power of impressing the human eye, and many other matters for which we must refer to Mr. Field, Mr. Owen Jones, and others, who have studied the subject and applied it.