and surfaces has brought about a definite use of geometrical terms': by young children which would have seemed very unnaturally mature a generation ago. But in color no corresponding advance has been made because there have been no generally accepted standards in color to correspond to the sphere, cube, cylinder, circle, ellipse and triangle in form, nor any means for measurements to take the place of the foot or metre for lengths and the divided circle for angles.
It is not expected that the children in the lowest grades will learn much of the science of color, but it is desirable that the teachers have such knowledge of it that they will not unconsciously convey to the children erroneous impressions which must be unlearned later in life.
More than two hundred years ago Sir Isaac Newton discovered that a triangular glass prism would transform a beam of sunlight into a beautiful band of color. If the prism is held in a beam of sunlight which enters a moderately lighted room, there will appear on the walls, ceiling or floor, here and there, as the glass is moved, beautiful spots in rainbow colors. If the room is darkened by shutters, and only a small beam of light is admitted through a very narrow slit and the prism properly adjusted to receive this beam of light, a beautiful band of variegated colors may be thrown on to a white ceiling or screen, and this effect is called a prismatic solar spectrum. A perfect solar spectrum once seen under favorable conditions in a dark room is a sight never to be forgotten.
The accompanying illustration shows the relative positions of the parts named. A is the beam of light as it enters the room. B is the triangular prism. The dotted lines represent groups of rays extending to the vertical band of colors indicated by the letters V for violet at the top, then blue, green, yellow, orange to red at the bottom.
The explanation of this phenomenon is that the beam of sunlight is composed of a great number of different kinds of rays,
