This view of the composition of the spectrum might have long remained unchallenged, had we not been able to apply to it a new mode of analysis. Though we cannot separate the _green_ rays of the spectrum into _yellow_ and _blue_ by the refraction of prisms, yet if we possessed any substance which had a specific attraction for _blue_ rays, and which stopped them in their course, and allowed the _yellow_ rays to pass, we should thus analyze the _green_ as effectually as if they were separated by refraction. The substance which possesses this property is a purplish blue glass, similar to that of which finger-glasses are made. When we view through a piece of this glass, about the twentieth of an inch thick, a brilliant prismatic spectrum, we find that it has exercised a most extraordinary absorptive action on the different colours which compose it. The _red_ part of the spectrum is divided into _two red_ spaces, separated by an interval entirely devoid of light. Next to the inner red space comes a space of bright _yellow_, separated from the red by a visible interval. After the yellow comes the _green_, with an obscure space between them, then follows the _blue_ and the _violet_, the last of which has suffered little or no diminution. Now it is very obvious, that in this experiment, the blue glass has actually absorbed the _red_ rays, which, when mixed with the _yellow_ on one side, constituted _orange_, and the _blue_ rays, which, when mixed with the _yellow_ on the other side, constituted _green_, so that the insulation of the _yellow_ rays thus effected, and the disappearance of the _orange_, and of the greater part of the _green_ light, proves beyond a doubt that the _orange_ and _green_ colours in the spectrum are compound colours, the former consisting of _red_ and _yellow_ rays, and the latter of _yellow_ and _blue_ rays _of the very same refrangibility_. If we compare the two red spaces of the spectrum seen through the blue glass with the red space seen without the blue glass, it will be obvious that the red has experienced such an alteration in its tint by the action of the blue glass, as would be effected by the absorption of a small portion of yellow rays; and hence we conclude, that the red of the spectrum contains a slight tinge of yellow, and that the yellow space extends over more than one-half of the spectrum, including the _red_, _orange_, _yellow_, _green_, and _blue_ spaces.