THE SEXTANT.] 



NAVIGATIONS-NAUTICAL ASTRONOMY. 



1113 



the astronomer will always surpass the power of the 

 artist ; and it must, therefore, be constantly the aim of 

 the former to make himself, as far as possible, indepen- 

 dent of the imperfections incident to every work the latter 

 can place in his hands. He must, therefore, endeavour 

 so to combine his observations, so to choose his oppor- 

 tunities, and so to familiarise himself with all the causes 

 which may produce instrumental derangement, and with 

 all the peculiarities of structure and material of each in- 

 strument he possesses, as not to allow himself to be mis- 

 led by their errors, but to extract from their indications, 

 as far as possible, all that is true, and reject all that is 

 erroneous. It is in this that the art of the practical 

 astronomer consists an art of itself of a curious and in- 

 tricate nature, and of which we can here only notice some 

 of the leading and general features." (See Herschel's 

 Astronomy, Lardner's Cyclopcedia. )* 



Returning now to the sextant : To understand the 

 way in which an angular distance is measured by this in- 

 strument, we must first assent to the following simple 

 optical principle that is, when a ray of light from a 

 luminous body falls upon a reflecting surface, and is 

 thence received by the eye, the incident ray or that 

 direct from the object to the reflector makes, with the 

 perpendicular to the surface of the reflector, drawn from 

 the point where it impinges on it, an angle equal to the 

 angle made with the same perpendicular and the reflected 

 ray, or that received by the eye. This property is briefly 

 expressed thus : the angle of incidence is equal to the 

 angle of reflection. It is the same with an elastic sphere 

 striking a smooth hard surface, as, for instance, a com- 

 i 11 marble, shot against a smooth wall ; if the marble be 

 shot perpendicularly, it will rebound along the same path, 

 and return to the hand, the angle of incidence and the 

 angle of reflection being nothing ; but if the marble be 

 projected obliquely, it will rebound on the other side of 

 the perpendicular ; and the oblique incident path, and the 

 oblique reflected path, will make equal angles with that 

 perpendicular. 



This principle being admitted, conceive the limb F to 

 be moved so that the attached index points to zero on the 

 graduated arc (the point A on the right in the figure 33). 

 In this position of the index and of the reflector B, if the 

 eye at E look through the upper or unsilvered part of the 

 horizon glass at C, and perceive a celestial object, such 

 as a star, it will at the same time also perceive the inut'je 

 of that star reflected from the silvered part of C. For as 

 the reflectors are, by hypothesis, parallel, and the star so 

 distant that two rays from it falling, the one on the glass 

 B and the other on C, must differ only insensibly from 

 parallelism, it follows that the ray from the star, reflected 

 at B and thence proceeding to C, from which it is again 

 reflected at , must proceed to the eye E in the same 

 direction as the direct ray from the star through the un- 

 silvered part of C. But this will be better illustrated by 

 a distinct diagram. 



Let d d (Fig. 35) be the position of the index glass 



Fig. 35. 



when the index points to 

 zero, which in the diagram 

 is marked by the letter e. 

 By hypothesis, the surface 

 of the half-silvered glass 

 C is parallel to that of <( d. 

 An object, R, so remote as 

 one of the heavenly bodies, 

 would be equally seen to 

 au eye at E in the direc. 

 tion of E D, parallel to 

 B K, as to an eye at B, in 

 the direction B R. If the 

 dotted line B N be per- 

 pendicular to d d, R B X 

 will be the angle of inci- 

 dence, and CBN the angle of reflection : the reflected 

 ray B C is now incident on C a surface parallel to d d ; 

 therefore the reflected ray C D must be parallel to R B, 



The reader my refer, on this lutiject, to the remark! on individual 

 error* of observation, &,;.. in the flection on Astronomy, u another source 

 of error, independent of the malruuitnt. ED. 

 VOL. L 



so that an eye at E will see the object directly through 

 the transparent part of C, and the image of it, after two 

 reflections, in the same direction ; or rather the object 

 and its image would become confounded and superposed. 

 Suppose now, while the eye is still looking at the object 

 R through the telescope E D, the index-limb be moved 

 from e to e' ; the reflector d d turning round with this 

 limb will take the new position d' d', and the image of R 

 will disappear, and that of some other object S, in refer- 

 ence to a ray from which, B C will still be the reflection, 

 will take its place. 



In this way, two luminous points, R, S, as, for instance, 

 two stars, or a star and an edge of the moon, or the 

 edges of the sun and moon, may be brought together ; 

 one of the two, R, being seen by direct vision ; the other, 

 S, after two reflections at the mirrors. 



The movement of the index from e to ', necessary to 

 bring the two objects R, S into contact, moves the re- 

 flector d d into the position d' d', and the perpendicular 

 B N into the position B N'. Now B N, by the above- 

 mentioned optical principle, bisects the angle RB C, and 

 B N', in like manner, bisects the angle SBC. Hence 



i(SBC-RBC) = JSBR. 



And since the angles N B N', e B e', measured by the arc 

 ee', are obviously equal, it follows that the arcee' mea- 

 sures half the angle S B R, formed by the two incident 

 rays SB, R B ; that is to say, it measures half the angular 

 distance of the two objects S, R. 



If, therefore, the arc A A, supposed to be 60, be 

 divided into twice that number that is, 120 equal parts 

 then, by considering each part as a whole degree, the 

 index at e' will show the number of degrees in the angu- 

 lar distance of S and R. In like manner, the degrees 

 and minutes will be shown if each division be subdivided 

 into 60 equal parts. This is the important principle in 

 the construction of the sextant. 



Besides the two reflectors, B and C, in figure 33, 

 several other glasses called screens or shades are at- 

 tached to the framework of the instrument, as shown at 

 K- and L. These are merely stained or coloured glasses 

 to be interposed in the path of the rays from luminous 

 objects, as the sun, and sometimes the moon, to reduce 

 the intensity of the glare, which might be too strong for 

 the sight, and, in the case of the sun, could not be en- 

 dured with a clear sky. The principal use of these 

 glasses, therefore, is to screen the eye ; but they ako 

 serve to distinguish the object from its image by differ- 

 ence of colour. The movable glass, G, is a microscope, 

 supported by a slip of metal turning about its extremity 

 a, so as to allow of its being brought over the divisions 

 of the Vernier a small and important scale attached to 

 the index, for the purpose of marking subdivisions too 

 minute to be engraved on the circular limb of the instru- 

 ment. We shall explain the Vernier presently. 



To USE TUB INSTKUMKNT. The plane of the quadrant 

 or sextant must always be held in the plane of the two 

 objects, of which the angular distance is to be taken ; it 

 is grasped by the handle H, usually with the right hand 

 (see Fig. 33, page ] 112), the other hand being employed 

 in moving and adjusting the index : when, as in common 

 quadrants, there is no handle, the instrument is held by 

 the framework itself. If the sun's altitude is to be 

 taken, the instrument is to be held in a vertical position, 

 the index set to on the limb, and the eye applied ta 

 the telescope, or removing this to the sight-vane, 

 which supplies its place, and directed through the 

 horizon glass to that part of the horizon which is ver- 

 tically under the sun. The index is now to be moved 

 forward till the limb of the image of the sun, which we 

 shall see to be gradually descending, just touches the 

 horizon : the observed altitude of that limb will thus be 

 obtained. 



The sight-vane may now be turned down and the tele- 

 scope introduced, which, by magnifying the image, will 

 render the contact more distinct. It is, in general, more 

 easy to get a contact, though with less precision, without 

 ;he telescope than with it, as the telescope greatly limits 

 ;hu field of view ; but after the index is adjusted to the 



7c 



