With this instrument we recommend in highest terms our new 

 Three-Screw Base, as shown in the illustration. It adds greatly 

 to the ease of manipulation and accuracy of observation for if the 

 bubble runs slightly off the center of its scale, due to temporal in- 

 fluences, as is frequently the case when directed to a new locality 

 in the field, it can be quickly restored to the normal position by 

 slightly turning the leveling screw nearest under the eyepiece. Each 

 of the leveling screws in this type of base works quite independently 

 of the other two and lends itself very nicely to this situation. To 

 level up with the 3-screw base, turn vial parallel to any two screws 

 and center bubble in the usual way. Now turn telescope 90 and 

 center bubble again with third screw alone. (See page 139) 



It has been contended that continued correction of this sort would 

 affect the H. I. One division on a 20" bubble represents Vioo % . One 

 who attempts to figure the one-ten-thousandth part of the distance 

 between the leveling screw and the vertical axis is dealing either with 

 fancy or prejudice certainly not with practical considerations. 



ADJUSTMENTS 



The Horizontally of the Horizontal Wire is deter- 

 mined by sighting on a plumb line. If the vertical wire does not 

 coincide, the studs of the reticule should be slightly loosened and 

 rotated the desired amount. It is not important, however, in this 

 or in any other leveling instrument that the vertical wire should 

 occupy the exact center of the field. 



Another method of making this test is to sight at some well 

 defined point on the distant horizon, after all of the other adjust- 

 ments are complete, and rotate the instrument on the vertical axis 

 over an angle equal to that of the field of view. If, in passing from 

 one side of the field to the other, the horizontal wire leaves the test 

 point, the reticule should be rotated as before until this deviation is 

 overcome. 



To Collimate the Sight Line in dumpies of our manufacture, 

 remove the saddle screws in the focusing pinion and withdraw the 

 pinion from the rack. This leaves the ocular tube free to move by- 

 hand. Focus the instrument on some distant point as one would a 

 "spy glass". Revolve the tube a half turn. If the horizontal wire 

 moves off the test point, correct half the error and repeat the oper- 

 ation, using the new position of the wire for the test point until the 

 requirements are satisfied. 



(0Bie difficult; will be experienced at first in keeping the telescope 

 properly focused without mechanical control, but a little patience 

 will overcome this. 



18 



