To adjust the instrument it is set up on the iron pipe and the pointer directed to some distant object.  All set screws, excepting those on the declination axis, are tightened.  Then the pointer is carefully turned through 180 deg. and if it is not again directed to the same point, it is not perpendicular to the declination axis.  When properly set it will describe a great circle.  With the declination axis in an approximately horizontal position the place where the pointer cuts the horizon is noted.  The declination axis is then turned through 180 deg., when the pointer should again cut at the same place.  Proper adjustment will cause it to do so.  It is desirable that the hour circle should read approximately zero when the declination axis is horizontal, but this is not necessary for a reason soon to be explained.  All these adjustments, once carefully made, need not be changed.

In using the instrument the hour axis can be directed to the north pole by the following method.  Point it approximately to the north star.  The pole is 1 deg. and 15 min. from the star on a straight line from the star to “Mizar,” the star at the bend of the handle in the Big Dipper.  Turn the hour circle into a position where the pointer can describe a circle through “Mizar.”  Only a rough setting is necessary.  Now turn the pointer so that a reading of 88 deg. 45 min. shows on the declination circle on that side of 90 which is toward “Mizar.”  When this is done, clamp both axes and turn the shafts in the base until the pointer is directed accurately to the north star.  It is evident from a study of the picture that the position of the small pointer which indicates the reading on the hour circle is not independent of the way in which the tripod or pipe is set up.  It would then be useless to adjust it carefully to zero when the pointer cuts the “zenith” as is done with a large equatorial.  Instead, the adjustment is made by setting the clock or watch which is part of the outfit.  The pointer is directed to Alpha, Cassiopiae, and the hour reading subtracted from 24 hours (the approximate right ascension of the star) gives the time which the clock should be set to indicate.  All of these settings should require not more than five minutes.

To find a star in the heavens, look up its declination and right ascension in an atlas.  Set the declination circle to its reading.  Subtract the clock time from the right ascension (plus 24 if necessary) and set the hour circle to the result.  The star will then be seen on the tip of the pointer.

To locate a known star on the map, turn the pointer to the star.  Declination is read directly.  Add the clock time to the hour reading to get right ascension.  If the result is more than 24 hours, subtract 24.

** A Ground Glass Substitute [255]

Ordinary plain glass coated with the following mixture will make a good ground glass substitute:  Dissolve 18 gr. of gum sandarac and 4 gr. of gum mastic in 3-1/2 dr.. of ether, then add 1 2-3 dr. benzole.  If this will be too transparent, add a little more benzole, taking care not to add too much.  Cover one side of a clear glass and after drying it will produce a perfect surface for use as a ground glass in cameras.  —­Contributed by Ray E. Strosnider, Plain City, Ohio.