#A803. Regulator Clock.# This clock has a first-class eight day movement with cut steel pinions. It is fitted with mercury compensation pendulum and electrical seconds contact. Price #$200.00# #Note.#--We can furnish free of duty to educational institutions astronomical precision clocks made by C. Riefler, Germany, and will be pleased to quote prices to interested parties. [Illustration: A 1200] #A1200. Simple Comparator.# (Fig. A1200.) Built up of #Micrometer Slide# M301 and #Microscope# M101, #with support# M310 #Fitted with Stage#. The stage is 150 mm. long and provided with clips for holding objects, such as spectrum photographs, scales, etc., and is fitted with illuminating mirror. Price #$45.00# #A1201. Small Comparator.# The instrument is intended for measuring spectra photographs, gratings, divided scales, or such objects which can be focused by the microscope and will allow rapid measurements of the highest possible accuracy. The measurement depends on the accuracy of the micrometer screw, which is cut and corrected with great care. The screw has a pitch of .5 mm. and diameter of 15 mm. The index head attached to the screw is of considerable diameter so as to allow the direct reading of .001 mm. The head is divided on solid silver in 500 parts, and carries two rows of figures indicating the first and second half of the mm. The full mm. are read by means of a scale in front of the instrument. The bed plate is heavy, of cast iron, and the guides are carefully scraped true within 0.001 mm. The carriage has a movement of 80 mm., is made of gun metal and fitted exactly to the guides; it is also provided with a second or top carriage with 40 mm. motion. The top carriage can be moved by hand and accurately set by means of a micrometer screw. The microscope is of variable magnifying power, focused by rack and pinion. Illumination for transparent objects is given from below by means of a plane mirror. The instrument is mounted on heavy supports, under an angle to make it convenient for the observer. The instrument is finished in first-class manner, and the iron bed plate heavily copper and nickel plated. The design of this machine was suggested by Prof. Edwin Frost of the Yerkes Observatory, where a number of these machines have been in constant use during the last five years. Careful tests have shown the screw accurate within .0003 of a mm. throughout the full length. Price #$175.00# #Note.#--The micrometer on microscope as shown in cut is not furnished with instrument. A tangent screw for revolving the eye piece with the spider thread can be attached at a cost of $5.00. [Illustration: A 1201] [Illustration: A 1202]