SKXTANT. 



SEXTANT. 



43* 



rently Mm in coincidence, the angle which they tubUod at the point 

 of meeting u exactly double the angle which U oonUined between the 

 plane* of the index and horison ghmm, if thw be supposed to be 

 prolonged until they meet Hence, U the index be at zero when the 

 (lane* are parallel, and if the arc be divided into half-degree* which 

 are numbered and subdivided at whole degrees, the arc actually read 

 off, after any tuch observation a* it here described, will measure twice 

 the inclination between the glissns (for this inclination is evidently the 

 same a* the angle between toe parallel position of the index-gloss and 

 its new position), and, in consequence of the optical principle above 

 mentioned, this will be the true measure of the angle between the 

 objects when they are so distant that the place of the eye and thu 

 intersection of the rays may be considered to be the same. The 

 sextant, therefore, furnishes the means of measuring the angle between 

 any two well-defined objects, in whatever direction they may be placed 

 (so that the angle does not exceed 140), and without requiring more 

 steadiness than is necessary for seeing the objects distinctly. K and r 

 an set* of dark glasses of varying intensity, which may be turned 

 before either the index- or horizon-glass when required ; o, a tangent- 

 screw for giving slow motion to the index-bar ; a, the reading micro- 

 scope and reflector. The handle below u visible in the drawing, and 

 there is a screw behind the collar of the telescope for giving an up-and- 

 down motion to the telescope, which alters its distance from the plane 

 of the instrument without affecting its parallelism thereto. 



The adjustments of the sextant may be divided into those which 

 more properly concern the artist and those which are to be attended to 

 by the observer. It should be distinctly understood that no other 

 instrument requires so much care and skill in its construction and use 

 as the sextant. There are scarcely half a dozen makers in England, and 

 probably not double that number in all Europe, who are fit to turn out 

 sextant; and though skilful sextant observers are somewhat more plen- 

 tiful, they are still exceedingly rare. The fabric of the sextant should 

 be strong and light. A blow which would not seriously damage a theo- 

 dolct or circle, where three or more readings correct petty nearly 

 the errors which arise from such injury, would entirely ruin a 

 sextant ; but while giving strength, the maker must be on his guard to 

 avoid weight. Troughton and Simms's double-frame sextants of 8 

 inches radius are a very good model, those of 10 inches are too large ; 

 single-frame sextants may be had of 7, 6, or 5 inches radius. All 

 these sixes have or may have the same telescope, glasses, and shades. 

 Those which we have hitherto seen seem to admit of some improve- 

 ment in their scheme. Mr. Dollond caste his sextants in one piece of 

 hard metal, which is an improvement, and very beautiful instruments 

 they are. The limb of the instrument should be perfectly plane, and 

 the axis on which the index-glass turns exactly perpendicular to it. 

 If a purchaser is sufficiently confident in his own judgment to rely on 

 that, rather than on the reputation of the maker, he should release the 

 clamp, observe whether the index-bar moves easily and without shake 

 along the whole arc, and also whether the vernier seems to press 

 equally in every position. The telescope should be of the best quality, 

 with a large aperture and of as short a focal length as these conditions 

 permit. It should show the edge of the sun and moon quite sharp 

 and distinct with the highest powers employed. The index -gloss is 

 generally the most faulty member of a common sextant; the faces are 

 not parallel, and consequently when rays fall very obliquely upon it 

 (in which case those reflected from the front surface are nearly as 

 copious as those from the silvered back), there are two faint and sepa- 

 rated images seen instead of one bright one. The artist tries his index- 

 glanes by the following test : The sun is viewed with a high power 

 after rery oblique reflection from the index-gloss, and those glasses 

 only are retained in which the image is single and sharp. The rejected 

 glasses find their way to the second-rate makers. The horizon-glass, 

 which is smaller and on which the rays fall at a larger angle, does not, 

 if imperfect, affect the images so much, but it too should be examined. 

 The shades or dark glasses, which are either interposed between the 

 gjsssns or placed before the horizon-gloss, are next to be examined. 

 The two images of the sun ore brought into proper contact, after 

 applying the highest power to the telescope and putting the proper 

 dark glass on the eye end. One of the lightest shades is then inter- 

 posed between the index and horizon glasses, and if the contact is 

 slightly disturbed, this shows the form of the shade to be prismatic. 

 The shade is turned in its cell until the effect is perpendicular to the 

 plane of the sextant. The corresponding shade before the horizon- 

 glass is now interposed and tried as the former one, and turned round 

 exactly as before, until its effect U also perpendicular to the plane of 

 the instrument. Leaving this Utter shade untouched, and substituting 

 the next dark shade in place of that first employed, the process is again 

 repeated, with this caution, that the error, if any, caused by the pris- 

 matic form of the last shade, U to be made exactly contrary to that of 

 the first shade, as well as perpendicular to the plane of the sextant 

 In this way, changing when necessary the dark glass at the eye end of 

 the telescope, all the dark shades are carefully examined and set right, 

 and it need not be said that those which are very prismatic or which 

 affect the sharpness of the images, are to be rejected! The index-glass 

 is now to be set at right angles to the plane of the instrument. This 

 is done by looking very obliquely in the index-glass, when the edge of 

 the limb or the divided arc seen by reflection will form an uninter- 

 rupted continuation to that seen directly, if the glass is perpendicular ; 



otherwise the arc will appear broken where the direct and reflected 

 image* meet ; by touching the screws seen in the figure, this error may 

 be corrected. Repeating the observation at different parts of the limb, 

 will show whether the axis has been set at right angles to the limb. 

 The horizon-glass U to be made parallel to the index-gloss by bringing 

 the two images of the sun to coincide, when, if there is any lateral 

 overlapping, it may be remedied by turning a screw (shown in the 

 figure) which is in the mounting of the horizon-glass. The index error, 

 a it is called, is of no importance whatever, though many oK 

 are afraid that when large it vitiates the observation. This is an idle- 

 prejudice, but one which the maker U compelled to respect. Having 

 ascertained the diameter of the sun by measuring it forwards and back- 

 wards, he sets the index to the proper angle and then files away a little 

 of one of the pins against which the index-glass is pressed, until he 

 brings the images nearly into contact. Contacts should now be formed, 

 first by turning the tangent-screw so as to separate the images, and 

 then by bringing them together. If the readings do not agree, the 

 centre moves too stiffly or the index-bar is too weak. Finally, the 

 telescope U to be set parallel with the plane of the instrument, 

 are two parallel wires in the focus of the telescope, which are to be 

 placed parallel to the instrument, and then as large an angle as possible 

 is to be taken between two distinct objects, making the contact exactly 

 in the middle of the wires. The contact must now be examined when 

 the objects are first at one side and then at the other of the field of 

 view. If they separate equally when at equal distances from the 

 centre, the position of the telescope is correct; if not, there ore 

 adjusting screws in the collar which carries the telescope by which 

 this error may be corrected. For some of these latter adjustments 

 mechanical means may be and sometimes are applied. The instrument 

 should however comply with the foregoing tests, which any one can 

 try who understands the use of the instrument. 



We will now suppose an observer equipped with such a sextant, who 

 wishes to make the best use of it. Before taking a series of observa- 

 tions, the index error should be ascertained. The telescope is fixed 

 and set to focus, and the parallel wires placed parallel with the plane 

 of the instrument. If the sun is visible, a dark gloss for the eye-end 

 of the telescope is selected, which shows him clearly, but without 

 distressing the eye, and the direct and reflected images are made 

 equally bright by moving the adjusting screw, which raises or depresses 

 the telescope. The contact is first mode, bringing the reflected image 

 apparently below the image seen directly, and the angle is rend 

 the supplemental or Kick arc : suppose this reading to be 33', which 

 consider positive or +. Then make the contact again with the 

 reflected image apparently above that seen directly (the reflected 

 image is that which moves on moving the index), and read off again : 

 suppose this is 32', which coll . It is evident that at the middle 

 position the two images coincide, and the index and horizon-gloss arc 

 parallel ; that is, the reading on the back arc is 30" when the glasses 

 we parallel. Hence 30'' must be added to every other reading to give 

 the true angle. If the algebraical signs ore used, the rule is very 

 simple : Add the two readings together and halve the result ; this with 

 its proper sign is the index correction to be applied to all observ 

 If the sun is not visible, the moon may be used in the some way. If 

 neither is to be seen, then bring the two images of a bright star, or the 

 sea horizon, or any distant well-defined object into contact, and the 

 reading, if on the back arc, is to be added, if on the forward are 

 be subtracted from all other observations. The sun is alway- 

 preferred when visible, and the observation is to be repeated before or 

 ifter correction. If the images overlap laterally, that is, if the 

 horizon-glass is not parallel to the index-glass, this must be previously 

 adjusted. The next caution is with respect to dark glasses. When it 

 is possible (as in observing altitudes of the sun at land in a met 

 liorizon, &c.) to make the observation with a single dork glass on the 

 eye-end of the telescope, without using any shade, this should always 

 l>e done, for the error of this dark gloss does not affect the contact at 

 all, and the distortion caused by it is not magnified, whereas any fault 

 in the dark shade between the index and horizon glasses produces 

 actual error in the observation, and the distortion is magnified subse- 

 meiitly by the telescope. The images are to be brought to equal 

 brightness by the screw which raises or depresses the collar carrying 

 the telescope, and that with considerable nicety. In observing 

 distances of the sun and moon, or altitudes of the sun at sea, dork 

 shades are necessary. The fainter object is to be observed directly, and 

 a proper shade interposed between the index and horizon glosses to 

 reduce the two objects to something like the some brightness. The 

 lina! equalisation is to be effected by the up-and-down motion of the 

 telescope. The accuracy of the observation depends o good deal upon 

 the nicety with which this equal toning of the two images is effected : 

 with a little experience this is readily learned. From the construction 

 of the sextant the faint object is easily viewed directly when the 

 brighter object is either above or on the right hand of thu fainter, but 

 not so if the bright object is to the left of the fainter, when the 

 liandle side of the instrument is uppermost. For these observations, 

 the instrument should have a second handle, which maybe applied 

 when wonted; perhaps when the distance between the moon and a 

 bright star or planet is to be observed, there may be no difficulty in 

 viewing the moon directly, after placing a dork shade before the 

 horizon-glass, though the light which enters the eye without passing 



