In 1909, we made for Prof. 1). A. Molitor a transit in which 

 the telescope was mounted revolvahly in an outer axis sheath similar 

 to the conditions that prevail in our telescopic alidades or in the 

 J. B. Davis Solar Transit * or in the Arrol-Tancreed Theodolite ** 

 but for quite a different purpose. This requirement would be 

 better served, however, by adopting the octilar focusing device as 

 prevalent in most European countries. 



The correct position of the horizontal wire in the exact equator 

 of the field of view is also important in the derivation of correct 

 vertical angles, particularly in mining theodolites where vertical 

 control is of paramount importance. 



Proceed as follow: Set a target, not less than 500 ft. 

 distant, coincident with the horizontal wire. The junction 

 can be made by slightly tipping the telescope, for in 

 making this test it is not necessary that the telescope 

 shall be in the horizon. The point sighted is only a 

 reference point, and the effects of refraction need not be 

 considered. 



Turn the instrument, if necessary, a degree or two 

 by use of the tangent screw and sight a scale, substanti- 

 ally placed, as near to the instrument as 

 the shortest possible sight. To see the 

 scale, such for instance as shown in 

 Fig. 36, will necessitate running out 

 the focusing system to the extreme limit] 

 by which an error in the placement of 

 the horizontal wire, with repect to the 

 optical axis will be registered but still 

 intangible. 



Revolve the instrument and invert 



the telescope. Carefully center the horizontal wire a 

 second time on the distant target, or whatever the test 

 point that may have been selected, and once more rack 

 out the focusing adjustment to take a second reading on 

 the scale. If the readings check, the adjustment is 

 satisfied. If not, there is no well defined rule to follow 

 in making the correction. Do not, however, correct back 

 half of the errorf but move the wire on, as if to increase 

 the error, by an amount at least equal to the interval. 

 Begin the test a second or a third time, and do not be 

 dismayed if an error is still in evidence. 



* Tra,n. <>/,i'<> /*,:. .SVx:., 1895; also Jour. Asso. Eng. Soc., Nov., 1896. 

 ** Trans. Inst. Mug. /:nxrs., Newcastle, Vol. 28, 1905, p. 642. 

 \J. A. h'itts in F.nf. News, Nov. 26. 1914, p. 1080, is in n mr on this point. 

 68 



36 



