telescope, below 64 in., as indicated in the table, he would be 

 required to apply the S. F. L. attachment and get control of dis- 

 tances ranging between 70 and 30 inches. 



With most S. F. L. attachments heretofore designed, it has 

 not been possible to sight less than 5 ft. with the instrument nor 

 over 4 ft. with the additional lens applied. Under these circum- 

 stances, collimation checks are impossible. 



The reference table gives for the 4^ and 5-inch Tachymeters 

 and Theodolites, minimum sights for the instrument and maximum 

 and minimum length of sights with the S. F. L. applied. The 

 longest sight with the secondary lens overlaps the shortest sight with 

 the instrument so that for average conditions, only one extra lens 

 need be employed, except in the case of the 5-in. inverting telescope 

 where two lenses are required to reduce the sight to 28 in. 



Let it be assumed that there exists some eccentricity. The 

 very fact that there is, will provide a ready means of adjusting the 



lens to the required purpose. In the position 1, both wires are assumed 

 to be displaced, but by continued rotation a correction can be made 

 for the horizontal or vertical wires as required. The S. F. L. will 

 rarely be used for leveling purposes as in position II. Sight 

 some object, therefore, with the vertical wire of the main telescope 

 at its shortest range and securely set all clamps. Apply the S. F. L. ; 

 re-focus the telescope and revolve the lens on the barrel of the 

 objective mount until the vertical wire appears to coincide with the 

 test point, as in cut III. For future use mark a check line against 

 an index on the barrel of the lens w ith the letter V and apply the 

 supplementary lens thereafter in this position for alignment work 

 or for reading horizontal angles. For leveling purposes the hori- 

 zontal wire can be checked up in the same manner, as in cut II and 

 the index marked with the letter H. Obviously this will occur at 

 a quadrant distance from the index line V. The argument is not 

 made complicated or difficult with adjusting screws. 



In manufacture we are enabled to rind a position for the S. V. ] .. 

 where it will be perfectly centered in both planes and cement it 

 there. The above suggestions are offered only in case of necessity. 



