98 Mr Adam on a Nautical Eye-piece for taking 



if not, its distance + from it is equal to the error of the ver- 

 tical adjustment, which may either be corrected by the screw, 

 or allowed for, like an index error of the sextant. 



To examine the lateral adjustment, screw the object tube 

 FB firmly into the sextant holder of the telescope by means 

 of the screw at y, or fix it steadily, by other means, in a hori- 

 zontal position, which is easily determined by the vertical ad- 

 justment of the level. Move the united eye and middle tube 

 a £ew degrees round in it to the right and left ; and observe 

 whether the bubble, formerly in the middle, now moves to 

 either end of the level tube. If it does not, the lateral ad- 

 justment is already made. If it does, correct the observed 

 motion by means of the adjusting screws at p. If this ad- 

 justment is not made, a slight deviation of the plane of the 

 sextant from the vertical plane, which the observer cannot de- 

 tect, when shut out from the horizon of the sea, may cause a 

 considerable error in the observed altitude. To prevent this, 

 let a plummet be suspended behind the plane of the sextant, 

 which will readily detect any deviation of the instrument from 

 the vertical plane. 



If EH, the middle tube of the telescope, be moved forward 

 or backward in the object tube FB, so as to place the object 

 glass a little too near, or too far from the cross hairs in the 

 centre of the field, the image of the observed object may thus 

 be brought nearer to or farther from the eye than the intersec- 

 tion of these cross hairs, without causing any apparent indis- 

 tinctness of the image. In this case, when the eye is slightly 

 elevated or depressed, it will cause the contact of the image 

 with the horizontal hair to appear either too close or too open, 

 and may thereby cause an error of one or more minutes in the 

 observation, according to the distance of the image on either 

 side of the cross hairs. 



To avoid this source of error, care must be taken to mark 

 on the middle tube EH a line t /3, to which the middle tube 

 should be moved, so that the image of a celestial object may 

 be formed exactly at the cross hairs ; for then, any elevation 

 or depression of the eye will cause no sensible change of the 

 apparent contact of the limb of the image with the horizont- 

 al hair. The proper distance of the object glass is a constant 



