96 Mr Adam on a Nautical Eye-Tube, for taking 



place at b, near the field of the telescope. The reduced dia- 

 meter of the part GK permits the upper side M of the level 

 tube to enter l-8th of an inch within the lower side of the eye 

 tube, and thus brings the bubble, seen directly through the 

 eye glass at A, as near as possible to the field of the tele- 

 scope. In the centre of the field two cross hairs of silk inter- 

 sect each other at right angles, the one horizontal and the 

 other vertical ; and the point of their intersection is adjusted 

 exactly into the line of vision through the telescope by means 

 of the screw nails c, d, acting on the diaphragm, the edge of 

 which, seen at e, is filed quite thin on the farther side, for the 

 purpose of more easily admitting the direct light of a lamp 

 through the aperture ab, to illuminate the cross hairs at night. 

 hi, qr, and st, are rectangular apertures in the frame of the 

 level tube, and tv is a reflector placed below st to illuminate 

 the spirits, and to show more distinctly the position of the 

 bubble. The lines^' and g, are painted on the level tube at 

 opposite extremities of the bubble, when it is in the middle ; 

 and, as the level is applied so that the line f is placed at the 

 focal distance of the eye glass, the eye end of the bubble can 

 be distinctly seen at^/J and at l-3d of an inch on either side 

 of it. When, therefore, M, the upper side of the level tube 

 is adjusted parallel to AB, the line of vision through the tele- 

 scope, if the eye end of the bubble be observed, and kept at 

 f, the line of vision AB must then be truly horizontal, or pa- 

 rallel to the horizon. In order to take the altitudes at sea by 

 a quadrant or sextant, furnished with this telescope and level, 

 which may be made capable of distinguishing 1CK, the obser- 

 ver should hold the sextant, as usual, in a vertical plane, pass- 

 ing through the celestial object whose altitude is required, 

 the telescope being horizontal, and then bring the reflected 

 image of the sun, moon, or star, into the field by the motion 

 of the index on the limb of the instrument, which, after some ex- 

 perience, he will generally be able to do upon the first or second 

 trial. When the celestial object is thus brought into the 

 field, and the near end of the bubble seen atj^in the level tube, 

 the observer should clamp the index on the limb, and, by 

 means of the tangent screw, while the near end of the bubble 

 is kept atj^ bring the lower limb of the observed object to 

 touch the horizontal hair passing through the centre of the 



