Eccentricity in the Eyepiece will not effect accuracy of 

 results but when the optical axis is not coincident with the collar-axis, 

 discrepancies will occur as explained later. Our method of centering 

 the eyepiece and dispensing with the objective draw tube does away 

 with the confusion of three or four sets of adjusting nuts protruding 

 from the telescope barrel, and maintains a more perfect equilibrium 

 while focusing on different points in the field of view.* 



The Bubble Adjustment proposes to secure parallelism be- 

 tween its tangential axis and the axis of the collars. Bring the bubble 

 to the center of its run over either set of leveling screws and carefully 

 reverse the telescope end for-end, in the wyes. Correct half the 

 error with the leveling screws and the other half with the adjusting 

 nuts on the bubble bracket. Restore the telescope to its original 

 position and note the location of the bubble. If the test has not 

 been carefully made or rectified, check the operation, as directed 

 above, over both sets of leveling screws. 



This process will secure parallelism between the axis of the 

 bubble and the contact points of the collars, quite irrespective of the 

 relative diameters of the collars themselves. When the collars become 

 unequal in diameter through wear, they are said to be conical and it 

 is not impossible that either collar may also become elliptical in sec- 

 tion through a slight variation in the density of the metal employed. 



Such a state of affairs cannot help but exert an insidious influ- 

 ence, not only on the collimation test at short distances, but upon 

 bubble adjustment as well. In the attempt to reconcile the bubble 

 axis with the optical axis through a variable medium of comparison 

 many serious and expensive errors have occurred.* * 



The Lateral Adjustment, for the bubble, or the "wind ad- 

 justment" as it is sometimes called, is intended to bring the longi- 

 tudinal axis of the level vial vertically beneath the axis of the tube. 



Revolve the telescope in the wyes so that the vial tube will 

 move in a short arc either side of the vertical plane. If the bubble 

 moves out of the center, correct the entire error with the lateral 

 adjusting screws in the lug at one end of the tube. The whole 

 error is corrected in this case because the amount has not been de- 

 termined by the process of reversion. 



Repeat the test by revolving the telescope so that the level vial 

 lies on the opposite side. If the bubble continues to run off center 

 in the same direction as before, this will indicate that the vial is 

 ground conically and not cylindrically. Such a discrepancy, how- 

 ever, is not a serious one and the side adjustment may be considered 

 as correct, in such a case, when the bubble runs off center an equal 

 number of divisions in the same direction at each side. 



./ in /-nz. AV, .. . \fr. ?S, 1914. Says " Here vuill be a 



<hift to the bubble when changing focus * * * on short sights * * * due to 

 the extraordinary length or overhang of the telescope.'" 



** Read anonymous article on "Sea Level Datum" in Ry. Enjf. & M. of IV.. 

 Chicago. Dec. 1914, recording a fruitless expenditure of $16,500 on 1462 mi. of 

 Precise control (') with ordinary initruments. 



8 



