Page:The American Cyclopædia (1879) Volume XI.djvu/530

 512 MICROMETER MICRONESIA AND MELANESIA Dollond's improvement consists in placing two half lenses side by side upon a sliding frame, and was called the divided object-glass mi- crometer. Other double-image micrometers are made of double-refracting crystals. This invention is ascribed to the abbe Rochon, rock crystal being the substance pfeferred by him on account of its transparency and hard- ness. Arago employed Rochon's micrometer in taking more than 3,000 of the diameters of the planets. A micrometer with double images was devised by Porro of Paris in 1842, which consists of a parallel plate of glass placed with- in the telescope so that part of the rays from the object pass through it and another part be- yond it. An inclination of the plate will pro- duce two images of the object, whose diameter may be measured by the amount of rotation given to the glass to produce certain alterations in the position of the image. A recent method for taking the positions and distances of stars, double or in clusters, is to photograph the tel- escopic field in collodion on glass, and then to measure the impression on the stage of an in- dependent micrometer constructed for the pur- FIG. 4. Rutherftird's Micrometer. pose. The great advantages of this method are that all the careful micrometer work can be done by an assistant who is not a professional astronomer ; that it can be done during day or night, and repeated at will, the photographs being imperishable ; and that when done it is most accurate. An instrument of this kind, which has undergone several stages of improve- ment, was designed and constructed by Lewis M. Rutherfurd of New York several years ago. A sketch of the instrument as now in use at his private observatory is given in fig. 4. It stands upon a tripod, which may be accurately levelled by the milled head screws attached to the feet. The photographic plate, p, to be mea- sured (five inches square), is clamped upon the circular glass stage &, which is supported by and revolves with the graduated position circle <7, the verniers being read by the reading glasses h h. The compound microscope a is directed perpendicularly to the plate, and can be moved in two directions at right angles to each other and parallel to the plate, on the slides e and d. The quantity of such movement is read by the microscopes c and 5 upon glass scales of equal parts, m. (The scale read by J is not seen in the figure.) The fractions of a division are read by the filar micrometer seen at the eye piece of c. The eye piece of the microscope a contains a cross of spider lines by which it is centred upon the image of any star to be measured. Microscope Micrometers. Most telescope micrometers may be used for mi- croscopes when the eye glass has considerable focal length. Objects of known diameter are used, as lycopodium seed, or wire whose diam- eter has been measured by winding it many times around a cylinder, and dividing the length of the cylinder by the number of turns. A convenient micrometer was constructed by Dr. FIG 5. FIG. 6. Wollaston, consisting of a scale, fig. 5, made of wires ^ in. diameter, which occupies the place of the object, and a lens of about -fa in. focal length in the cap of the instrument. The ob- ject, placed beneath this between two glass slides, is moved laterally across the field by means of the milled head a. (See MICROSCOPE.) MICRONESIA AND MELANESIA (Gr. putpd^ small, [i&ag, black, and vrjaos, island), terms derived from the size and complexion of the inhabitants, and applied by some geographers to arbitrary divisions of the islands of the Pacific ocean. These divisions arc both com- prehended in the better defined, more conve- nient, and better understood terms Australasia