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 the upper face of the prism P3. Then if the prism P4 is cemented to P3, a sharp image of such lines of the solar spectrograph as are visible in the field of view will be seen in the eyepiece. If the stellar spectrograph is viewed in the focus of O2 and the converging rays are reflected by the prism P2 to P4, no image would be seen in the eyepiece, for the rays would pass out directly through the parallel glass plate which is formed by the cementing together of the prisms P3 and P4. EB1911 - Micrometer - Fig 23.png From Zeitschr. für Instrumentenkunde, by permission of Julius Springer, Berlin. 23.

But if the cemented face of P4 is silvered, then the lines of the stellar spectrogram would be seen in focus of the eyepiece and the image of the solar spectrogram h would be obliterated. Therefore, if one-half of the cemented face of P4 is silvered, it becomes possible to view, side by side, one-half of the image of the solar spectrograph formed by O1 and one-half of the image of the stellar spectrograph formed by O2. A prism half silvered in this way is provided, which enables the observer to compare the equality of scale of both photographs. If, for example, it is found that the image of the solar spectrograph is the larger of the two it becomes necessary to adjust the object glass O2 farther from the stellar spectrograph. This has the effect of forming the image of the latter farther from the observer’s eye, and so it becomes necessary to turn the handle of the rack-pinion V in such a way as to move the prisms P3 and P4 nearer to P2 till the lines of the stellar spectrograph are again sharply in focus. The effect of turning the pinion V is, of course, to displace the focus both of the solar and stellar spectrographs in the field of the eyepiece, but this displacement, is easily restored by focussing screws O1 and O2. By successive adjustments of this kind condition (3) can be accurately realized.

These three adjustments having been made, the prisms P3 and P4 are removed and replaced by another prism in which the silvering is arranged as in fig. 24, where the hatched lines denote the silvered surfaces. The narrow tongues of the silvered surface will now reflect corresponding parts of the star-spectrograph, and will obliterate corresponding parts of the solar spectrograph—as shown in figs. 25 and 26. Fig. 25 shows the stellar and solar lines of the two spectrograph’s in coincidence, whilst the metallic lines of comparison are non-coincident. Fig. 26 shows the metallic lines of comparison in coincidence whilst the solar and stellar lines are non-coincident. It is obvious that these two conditions can be produced at the will of the observer by simply turning the screw S, and that the difference of the readings of the screw-head, which are required to reproduce the two conditions in question, gives a measure of the displacement of the stellar lines relative to the solar lines. If then the screw-value in kilometres per second is known for the neighbourhood of each of the comparison lines employed, the radial velocity of the star can be independently derived directly from coincidences made in above manner in the neighbourhood of each comparison line. For the special purpose of determining the solar parallax this instrument has been used in a most refined and perfect manner by Dr Halm at the Cape of Good Hope (Annals of the Cape Observatory, vol. x. part 3).

EB1911 - Micrometer - Fig 25.png From Zeitschr. für Instrumentenkunde, by permission of Julius Springer, Berlin. . 25.

EB1911 - Micrometer - Fig 26.png From Zeitschr. für Instrumentenkunde, by permission of Julius Springer, Berlin. . 26.

Double Image Micrometers are described in the article (q.v.).

MICRONESIA (from Gr. , small, and  , island), one of the three great divisions of the oceanic islands in the central and western Pacific. Lying to the north of Melanesia, it embraces the following groups:, , , and. See articles under these headings, and (section on Islands).

The Micronesian islanders form in the main a branch of the Polynesian race, but distinguished from it by well-marked differences in appearance, language and institutions. Many of the islanders, however, show signs of hybridism. The proximity of Japan and the Philippines on the west, and of the Papuan