The above question having been set before me by my friend Prof. P. G. Tait, and in such a guise that its practical solution, if amounting to anything sensible, might have astronomical applications, I set myself to examine it with the highest dispersion power in my possession, viz., a fine Rutherfurd Diffraction grating, of 17,296 lines to the inch, ruled over a surface 1·6 inch square; a telescope 52 inches long, with magnifying powers from 20 to 50; and a collimator 32 inches long, armed with a very substantial slit-apparatus by Mr Adam Hilger, and some other fittings.