The solar spectrum is so commonly composed to have been mapped with completeness, that the statement that much more than one-half its extent is not only unmapped but nearly unknown, may excite surprise. This statement is, however, I think, quite within the truth, as to that almost unexplored region discovered by the elder Herschel, which, lying below the red and invisible to the eye, is so compressed by the prism that, though its aggregate heat effects have been studied through the thermopile, it is only by the recent researches of Capt. Abney that we have any certain knowledge of the lines of absorption there, even in part. Though the last-named investigator has extended our knowledge of it to a point much beyond the lowest visible ray, there yet remains a still remoter region, more extensive than the whole visible spectrum, the study of which has been entered on at Alleghany, by means of the linear bolometer. The whole spectrum, visible and invisible, is powerfully affected by the selective absorption of our atmosphere and that of the sun; and we must first observe that could we get outside our earth's atmospheric shell, we should see a second and very different spectrum, and could we afterward remove the solar atmosphere also, we should have yet a third, different from either. The charts exhibited show: 1st. The distribution of the solar energy as we receive it, at the earth's surface, throughout the entire invisible as well as visible portion, both on the prismatic and normal scales. This is what I have principally to speak of now, but this whole first research is but incidental to others upon the spectra before any absorption, which though incomplete, I wish to briefly allude to later. The other curves then indicate: 2d. The distribution of energy before absorption by our own atmosphere. 3d. This distribution at the photosphere of the sun. The extent of the field, newly studied, is shown by this drawing [chart exhibited]. Between H in the extreme violet, and A in the furthest red, lies the visible spectrum, with which we are familiar, its length being about 4,000 of Angstrom's units. If, then, 4,000 represent the length of the visible spectrum, the chart shows that the region below extends through 24,000 more, and so much of this as lies below wave-length 12,000, I think, is now mapped for the first time. [Illustration: FIG. 1.--PRISMATIC SPECTRUM.] We have to pi = 12,000 relatively complete photographs, published by Capt. Abney, but, except some very slight indications by Lamansky, Desains, and Mouton, no further guide.