#A1204. Comparator.# For Plates 8x10 inches. This machine was constructed in accordance with suggestions given by Dr. Frank Schlesinger, Director of Allegheny Observatory. The ways are carefully straightened to within 0.002 millimeter. The carriage is moved by two racks and pinions and has a large handle on each side. Two concentric circles are fitted to the carriage, the inner circle carries the plate with the film in a fixed plane, no matter what the thickness of the glass may be. Both circles are provided with clamps and tangent screws, so that each one may be clamped and adjusted independently. The outside circle carries four index points 90 degrees apart. One of these marks is made adjustable. These four marks serve to turn the plate exactly 90 degrees so as to measure rectangular coordinates. The guide carrying the measuring microscope is adjusted exactly at right angles to the ways of the bed plate. The carriage supporting the microscope is moved by rack and pinion. The microscope is arranged to tilt, so as to view either the plate or the scale above. The eye piece is provided with a reversing prism. Measurements are made on the scale divided in millimeters. The smaller measurements, to 1-4000 of a mm., are made by the micrometer. The micrometer is fitted with sliding eye piece and counter for full revolutions. The scale is not marked in the usual way with single lines, but each millimeter is marked with a double line. This double line allows the use of one single spider thread, which is preferable to use for bisecting the star, and the double line on the scale allows a clear setting with the single spider thread. The stage is fitted with an adaptor for holding plates 4x5 inches. Price #$450.00# This machine has been in use during the last years at the Yerkes Observatory and has been found very convenient and satisfactory in every way. A graduated circle can be provided if desirable. Such division on solid silver, with verniers reading to 1-100 of a degree will increase the cost $50.00. #A1205. Comparator#, for plates 4x5 in. of the same design as A1204. Price #$300.00# [Illustration: A 1206] #A1206. Comparator.# A larger machine of the same design as A1204 with scale of 45 cm. length, divided in double lines to 0.25 mm., and with two carriages, one for spectrum plates of 18 in. length and the other with divided circles for star photographs 12"x14". This instrument was constructed for the Solar Observatory of the Carnegie Institution at Mount Wilson, California. [Illustration: COELOSTAT. As built for the United States Naval Observatory after design furnished by Mr. W. W. Dinwiddie.] [Illustration: ARMILLARY SPHERE. Built after design of Prof. J. F. Lanneau, Wake Forest College, N. C.] _NOVEMBER, 1905_ Wm. Gaertner & Co. Astronomical, physical and physiological Apparatus 5347-5349 LAKE AVENUE CHICAGO [Illustration]