4514 HYDROGRAPHIC MANUAL PaGE 364 



The mirrors of the hydrographic sextant are sHghtly smaller in size than those of 

 the navigating sextant. Hydrographic sextants are either not provided with shade 

 glasses or they are removed before use. 



Hydrographic sextants are often constructed so that the maximum angle that 

 may be read on them is slightly larger than on the navigating sextants; depending on 

 the construction, angles between 144° and 151° can usually be read. 



4514. Sextant Mirrors 



There are two mirrors on each sextant. The index mirror A (fig. 76) is held in an 

 adjustable frame on the arm AE and is so called because it is attached to the index 

 arm and rotates with it. It is the larger of the two mirrors and its entire area is a 

 reflecting surface. The other mirror B is called the horizon mirror because it is through, 

 or above, it that the observer sights at the horizon when measuring the altitude of a 

 celestial body. This mirror is considerably smaller than the index mirror. It is also 

 held in a frame, which is provided with two adjustments. Formerly, this mirror was 

 a glass of which only the lower half was a reflecting surface, the observer looking 

 through the clear upper half directly at one of the objects. In the Coast and Geodetic 

 Survey, only the reflecting portion of the glass is used, the observer sighting over it 

 at the direct object. 



Formerly, all mirrors were of glass, the reflecting surface consisting of silver de- 

 posited on the back of the mirror. 



A fresh chemically deposited silver surface has an exceptionally high reflectivity, 

 approximately 90 percent, but it tarnishes readily and is easily scratched. When the 

 reflecting surface is on the reverse side of the glass and the light rays must pass through 

 the glass twice, the reflectivity is reduced in varying amounts, probably resulting in an 

 average reflectivity of 80 percent. 



. The reflectivity of highly polished stellite is 67 percent and that of highly polished 

 stainless steel is approximately 61 percent. 



a. Glass mirrors. — Stellite mirrors are used almost exclusively in hydrographic surveying in the 

 Coast and Geodetic Survey, but due to the higher reflectivity of the best glass mirrors their use may 

 still be desirable for some purposes. 



Glass mirrors must be optically flat and their two surfaces must be v^'ithin 2 seconds of parallelism. 

 The glass reflects from its front surface as well as from the mirrored surface. If the two surfaces are 

 parallel the reflected images from the two will coincide in the telescope to produce a single image, 

 but if the two surfaces are not parallel there will be two images or a distorted image which may appear 

 as though it were out of focus. If the reflected object is a star it will appear enlarged or elongated. 

 Sextant mirrors with these characteristics should not be used for accurate observations but should 

 be returned to the Washington Office with a report of the defect. 



The error due to a prismatic index mirror will vary with the size of the angle measured. The 

 error due to a prismatic horizon mirror, howeyer, is a constant and is included in the index correction. 

 Consequently, if the effect of the latter is not otherwise disturbing it need not be replaced. 



When a glass mirror is inserted in its holding frame, the screw at the back of the frame must not 

 be tightened so much as to introduce a torsion in the glass. 



The greatest disadvantage of glass mirrors is the rapid deterioration which takes place in the 

 reflecting surface from exposure, dampness, and especially salt. Since hydrographic sextants must 

 be used at sea in all conditions of weather, they are constantly exposed to salt spray, particularly those 

 used in launch and smaU-boat hydrography. After use the entire sextant and particularly the mirrors 

 should be carefully dried and cleaned. But the spray that finds its way around the edges of the 

 mirror cannot be easily removed and thus is in a position to attack the mirrored coating. 



A British manufacturer has recently devised round glass mirrors hermeticall}' sealed in frames 

 to prevent the possibility of spray reaching the mirrored surface. These mirrors are silvered, copper 

 plated, and painted with special resistant paint and are then mounted in such a way that neither air 



