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A corresponding hand-drawing, for which M. Thollon received in 1886 the Lalande Prize, exhibits, not the diffractive, but the prismatic spectrum as obtained with bisulphide of carbon prisms of large dispersive power. The grating is an expensive tool in the way of light. Where there is none to spare, its advantages must be foregone.
Both Rutherfurd's and Rowland's gratings are machine-ruled, and reflect instead of transmitting the rays they analyse; but Rowland's present to them a very much larger diffractive surface, and consequently possess a higher resolving power.
Spectra are, however, formed by diffraction, wherein the distribution of both heat and light is different from that produced by the prism. These diffractive spectra have been examined with great skill by Draper and Langley. In the prismatic spectrum the less refrangible rays are compressed into a much smaller space than in the diffraction spectrum.
The magnificent sunglows due to the diffractive effects on light of the vapours and fine dust flung in vast volumes into the air, and rapidly diffused all round the globe, betokened an atmospheric condition of all others the most prejudicial to delicate researches in the solar vicinity.
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