R R are rays coming from the source of light, be it sun light or the electric light, and an image of the one or the other is formed by a lens L₁ on the slit S₁ of the collimator C. The parallel rays produced by the lens L₂ are partially refracted and partially reflected. The former pass through the prisms P₁, P₂, and are focussed to form a spectrum at D by a lens L₃. D is a movable screen in which is an aperture S₂, the width of which can be varied as desired. The rays are again collected by a lens L₄, and form a white image of the surface of the last prism on the screen E. If the light passing through S₂ is alone used, the image at E is formed of practically monochromatic light. Part of the rays falling on P₁ are, as just said, reflected, but as it and the refracted part are portions of the light passing through the slit S₁, they both must vary proportionally. If then we use the reflected portion as a comparison light to the spectrum colours, the relative intensities of the two, though they may vary intrinsically, will remain the same. The rays reflected from P₁ fall on G, a silver or glass mirror, and, by means of another lens L₅, also can be caused to form a white patch on the screen E, alongside the patch of colour. At M, or anywhere in the path of the beams, an electro-motor driving a sector with apertures which can be opened or closed whilst rotating, is placed, and the illumination of either beam can be altered at will. To obtain a large spectrum on the screen E, all that is necessary is to interpose a lens of fairly short focus in front of L₄, when a spectrum of great purity and brightness can be formed. If it be required to measure the width of the slits S₂ (which we shall see further on is often necessary), a small lens of short focal length placed behind L₄ and near the slit will cast a magnified image on E, and by means of a scale placed there, the widths of each slit, if there are more than one, can be read off on the scale by bringing them successively into the same colour. [Illustration: FIG. 5.] Originally the comparison light was a candle, and it answered its purpose fairly well, and for obtaining absolute measures is convenient at the present time. Fig. 5 will show its arrangement, but as both the candle and the electric light may vary independently of each other, it will be seen that for merely the comparison of the different spectrum colours, the previous arrangement is the better. In both cases the two beams--the direct and the comparison--may be made to cast shadows by placing a rod in their path, the shadow cast by one light is then illuminated by the other light. By moving the rod towards or from the screen the shadows can be brought side by side.