This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1912 edition. Excerpt: ... spectrum. At each coincidence a reading was made, care being taken to proceed in one direction only. As there are four spectra more or less in coincidence, it is easily possible to select the wrong line. By temporarily screening off the reflection from one mirror, however, and then marking the line ...
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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1912 edition. Excerpt: ... spectrum. At each coincidence a reading was made, care being taken to proceed in one direction only. As there are four spectra more or less in coincidence, it is easily possible to select the wrong line. By temporarily screening off the reflection from one mirror, however, and then marking the line desired of the remaining pair by the cross-hairs of the telescope, mistakes are usually avoidable. In the extreme blue and red it is often difficult to see the lines clearly in the superimposed spectra, so that the data for the B and G lines are not quite so trustworthy. In fact, the ellipses come out sharpest when but a narrow strip of the grating is used; whereas the resolving power of the grating or general sharpness of lines increases with the breadth of the grating. If the whole grating is used (an intense spot of sunlight focussed on the slit) the adjustment must be very perfect and the surfaces quite true to bring out the ellipses; this is not the case if parallel rays fall on the slit and the whole divergence in the collimator is due to the diffraction of the slit. With optical plate-glass none of these difficulties would appear. Fig. 51.--Graphs showing displacement of opaque mirror (micrometer motion) to place center of ellipses in different parts of spectrum, for case of different angles of incidence, /. Apart from drawbacks of this nature, which require patience for their removal, the measurements are quite satisfactory as a whole, as may be seen in fig. 51(a). In table 32 I have given the observed results, N'c being the arbitrary drum-reading of the micrometer and its equivalent in centimeters, from which Anc = N'c--(n'c)e is deduced, relatively to the position (n'c)e corresponding to the E line. Different values of (n'c)e in the...
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