For a sextant enthusiast, to own a box sextant after handling full sized instruments gives a lot of pleasure. These dainty and ingenious little instruments, measuring only 75 mm in diameter, can be slipped into a pocket, but be quickly brought out to make a reading with a precision of around an arc-minute. They are equiped with shades that allow sun and moon observations and some models come with a small telescope. They are especially handy for taking horizontal angles on land.
Unfortunately, there are many so-called reproductions on the market, and it may not be easy for the inexperienced to distinguish them from the genuine article. One should look for fine, usually straight knurling on the adjusting knobs, crisp edges to the index arm and magnifier arm, sharp corners on the bracket that holds the latter, smooth, ground edges to the mirrors and sweet operation of the various adjustments. A bright sky should just be visible through the darkest shade (usually red) and should not be visible at all when used in combination with the second shade (usually blue). The two together should allow comfortable viewing of the full sun.
The scale is usually divided on silver, each degree being divided into halves, with a vernier reading to one minute. The arc radius is about 46 mm. Look for crisp regular engraving of the numerals rather than the uneven, stamped numbering with rounded edges often seen in reproductions. The numbering on the vernier usually omits the fives, and a scale with crowded numerals reading 5, 10, 15 etc is quite likely to be a fake. The scale is read from the centre of the index arm, the opposite of a nautical sextant, which is read towards the centre.
A maker’s name may be of little help. There must be thousands of so-called “Stanley” and “Kelvin-Hughes” instruments around, the latter usually dated 1918, nearly thirty years before the real company came into being. Stanley was a highly respected firm of instrument makers and suppliers, founded in 1853 and trading well into the second half of the twentieth century. Older instruments are engraved “Stanley, London” in beautiful copperplate, though they were probably made (and later labelled) by Heath and Co, with whom Stanley eventually merged in 1926. There are box sextants bearing the name of Elliott Brothers and many other nineteenth century instrument makers and most of these are genuine, but it is important to look at the general workmanship as well as the name. Stanley is the name that has been most abused.
The first photo shows a genuine box sextant by Stanley, alonside its big brother from the same period, a vernier sextant of eight inches radius by Crichton of London, dated to about 1850. The lid unscrews to expose the controls, and is screwed on to the base to act as a handle in use.
The next photo shows the underside of the browned bronze base of the instrument. The nib is used to slide the cover aside. This opens the slot through which shades emerge when not in use, as will be seen in a later picture.
The general view below shows some of the main features. Rotating the control knob moves the index arm and the vernier over the scale while rotating the index mirror. Note the fine knurling and the crisp edges of the pin holes in the central screw. The magnifierarm, its bracket and the index arm also have crisp edges and the screw slots are narrow, the screw heads polished. Sliding the nib brings the peephole into position and a knurled screw is provided above the peephole to attach the telescope when it formed part of the kit (in some makes, the telescope screwed into the hole). Next to this screw is the mirror-adjusting tool, which screws into place. The levers for bringing the shades into and out of position are to the bottom left of the photo. When they are not in position, they project through the slot described in the preceding paragraph. At the top end of the scale can be seen the two square-headed screws which are used to adjust out side error.
The next photograph shows many of these features from a different viewpoint, that also shows the window opposite the peephole and the head of the screw used to adjust for index error.
For those of you who dare not take their instruments apart, in the next picture I have done it for you, by removing three screws from the periphery of the base plate. The shades are raised out of the way. In use, the head of a limit screw ensures their correct position. The index mirror, its bracket and keeper are mounted on a bearing and are rotated by the toothed sector or rack by means of the control pinion, the business end of the control knob. The horizon mirror sits on a base that can be rocked by means of two spring loaded screws to remove side error and the sub-base below it can be rotated by a further spring loaded screw to remove index error.
The next picture shows another view of the interior to show more details of the horizon mirror arrangement.
Finally, this picture, showing the instrument in use, gives another impression of its size.
If you enjoyed reading this post or found it helpful, do let me know and if you have a “doubtful instrument” I will be happy to view a photo and advise.