ps 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]

#A1202. Comparator for Measuring Spectra Photographs, Scales, Etc.# Range
50 cm. The microscope travels on carefully straightened ways, and is moved
by a screw of 1 mm. pitch. The screw head is faced with a silver band and
is divided in 1000 parts. The handle for turning the screw is placed on the
le