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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.

From Zeitschr. für Instrumentenkunde, by permission of Julius Springer, Berlin.
Fig. 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
 From Zeitschr. für Instrumenten-
 kunde by permission of Julius
 Springer, Berlin.

Fig. 24.

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).

From Zeitschr. für Instrumentenkunde, by permission of Julius Springer, Berlin.
Fig. 25.

From Zeitschr. für Instrumentenkunde, by permission of Julius Springer, Berlin.
Fig. 26.

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

MICRONESIA (from Gr. μικρός, small, and νῆσος, 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: Mariana, Pelew, Caroline, Marshall and Gilbert. See articles under these headings, and Pacific Ocean (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^[1] on the west, and of the Papuan

1. There are authenticated instances of Japanese junks, with living people in them, having been found in various parts of the North Pacific. In 1814 the British brig “Forester” met with one off the coast of California (about 30° N. lat.), with three living men and fourteen dead bodies on board. In December 1832 a Japanese junk arrived at the Hawaiian Islands with four of the crew living. If these junks could cross the Pacific in the latitude of Hawaii it is not at all unlikely that others running in a south-easterly direction would reach some of the many atolls which stretch over about 35° of longitude, forming the Caroline and Marshall archipelagoes.
    
   The traditions of the Gilbert Islanders tell us that their islands were peopled from the west and also from the east. Those who came from the east are expressly said to be from Samoa. Those from the west were more numerous than those from the east. There are also traditions of the arrival of other strangers at some of these islands. On the island of Peru, in the Gilbert group, in 1869 there was still the remnants of a large proah which, from the description given, appears to have been like those used in the Indian Archipelago.