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]

Spectrometers and Accessories

#Cat. No. L4020 LABORATORY SPECTROMETER.# This instrument is very rigid and
accurate in construction and every part of its design has been carefully
considered. The circle has a diameter of 15 cm. and the two verniers read
to 20 sec. It is fitted with protecting plate which adds greatly to the
value of the instrument if put in the hands of students. The graduation is
on solid silver. The verniers are fitted with adjustable magnifying
glasses, the telescopes have an aperture of 25 mm., focal length of 200 mm.
A hori