47 



Arrangement for Offsetting at Bight Angles. 



The most common off-set with the transit is one at 90 to the line of sight. 

 Several methods have been proposed for doing this without disturbing the telescope. 



Messrs. C. L. Berger & Sons have a very neat one ; it consists in simply per- 

 forating the horizontal axis, so that by drawing the head back fifteen or twenty 

 inches from one end of the axis, the eye may be placed so that the eye, the hori- 

 zontal axis of the telescope, and a rod set beyond, may be readily placed in the 

 same straight line, at right angles to the line of sight of the telescope, no matter 

 at what altitude the telescope may be pointing. 



Jn oft-setting by the arrangement proposed above, the rod is made plumb by 

 lining it with the plumb-line of the instrument itself. The advantage of this method 

 is, that it holds equally well for any inclination of the telescope. The disadvantage 

 is, that the engineer is obliged to leave the eye-end of the telescope at each off-set 

 made. Where the engineer is willing to bring his telescope nearly level before 

 each off-set is made, Messrs. 0. L. Berger & Sons will adapt a eimple combination 

 of two prisms to the telescopes, by which the rod may be made plumb, and set 

 at an angle of 90 to the line of sight. 



Setting Up. 



In setting up a transit, push the iron shoe of one leg firmly into the ground, by- 

 pressing on the other two legs near the tripod head. Having secured a firm found- 

 ation for this leg, separate the other two legs, at the same time drawing the tripod 

 head toward you. Then set the two remaining legs in the same manner as the first 

 one. If the ground is pretty level, merely noticing that the tripod feet are equi- 

 distant, will insure that no unsightly appearance will be ^iven to the leveling screws. 

 If the ground is uneven, however, nothing but practice can produce a graceful 

 position of the instrument. The plumb-bob attached to the instrument should 

 swing within say half an inch of the point on the ground, and the plate on which the 

 leveling screws rest, if possible, should be approximately horizontal, when this 

 stage is completed. 



Now with the level screws not tightened up, after leveling approximately, 

 bring the plumb-bob exactly over the point on the ground, bv moving the body of 

 the instrument on its shifting head. Then complete the leveling of the instrument, 

 and it is ready for work. 



The Adjustments of the Transit. 



In a theoretically perfect transit instrument, the following points are established : 



1. The object and eye-glasses are perpendicular to the optical axis of the 

 telescope at all distances apart. 



2. The line of collimation coincides with the optical axis. 



3. The line of collimation is parallel to the telescope level. 



4. The line of collimation passes through, and is perpendicular to the horizontal 

 axis of revolution. 



5. The vertical circle is perpendicular to the horizontal axis. 



6. The center of its graduated arc lies in the horizontal axis. 



7. The arc reads zero when the line of collimation is perpendicular to the ver- 

 tical axis of the upper plate. 



8. The pivots of the horizontal axis of the telescope are circles. 



9. The bearings for these pivots are of the same diameter or otherwise exactly 

 similar. 



10. The line of collimation moves in a plane perpendicularly above the center of 

 the horizontal graduated circle. 



11. The horizontal axis is perpendicular to the axis of the upper plate. 



12. The upper plate is perpendicular to its axis. 



13. The radial lines which form the graduations of the circle and verniers are 

 equi-distant at the same distance from the axis of the upper plate, and pass through 

 this axis. 



