This small volume (40 pages) collects two papers published by the author in April and May of 1888, in the American Journal of Science. Bound together, they were submitted as the dissertation research for his PhD in physics at Johns Hopkins, under the direction of Henry Augustus Rowland. Bell's work was directed at measuring the "absolute" wavelength of the second line in the sodium doublet at 5890 and 5896 Angstroms in the "normal" solar spectrum. The work employed four different of Henry Rowland's recently-developed concave diffraction gratings. Rowland used Bell's results for the absolute wavelength of the D2 sodium line as the anchor point to assign values for the "relative" wavelengths of all of the other spectral lines published in his Photographic Map of the Normal Solar Spectrum (1888) and Table of Solar Wave-Lengths (1898).