1882.] the designs of the late Rev. Dr W. Pearson. 235 



circular rim at right angles to the plane of the circle, and this rim 

 can be clamped to the revolving cone of the azimuthal motion. 



The vertical circle can be clamped either to the revolving cone, 

 or to the alidade. 



The telescope is capable of adjustment so that its line of colli- 

 mation may be parallel, or may make a small fixed angle with the 

 axis of the revolving cone, by means of a microscope fixed to that 

 cone and viewing on its cross wires a small dot on a slightly adjus- 

 table piece of platinum borne by a short arm projecting from the 

 tube of the telescope. 



This instrument, after the death of the designer, passed into 

 the hands of a country gentleman, who sold it to the Rev. N. S. 

 Godfrey, Vicar of St Bartholomew, Southsea, from whom I pur- 

 chased it about Easter 1881. With the sanction of the Museums 

 Syndicate I have added it by gift to the Plumian Professor's 

 collection of apparatus, having found it to be a very useful ex- 

 ample of a well graduated and adjustable altazimuth telescope, or 

 zenith instrument. 



The vertical circle is graduated to every 5' of arc and is read 

 by three verniers to every 5", its radius is about 8 inches. 



The horizontal circle is graduated to every 10' of arc and is 

 read by two verniers to every 10", its radius is about 6 inches. 



I have determined the errors of excentricity of both alidades in 

 magnitude and direction, as also the angles between their arms 

 by discussion of the readings of each circle in three or more posi- 

 tions, representing each reading in the form given in Chauvenet's 

 Astronomy, vol. II. § 27. 



These errors are small. The magnitudes of the errors of excen- 

 tricity are 8""487 for the vertical circle, and 14"'046 for the hori- 

 zontal circle, estimated in arcs of the respective graduated circles, 

 equivalent respectively to j$$qq and jo/oVo °f au mcn - It is un- 

 necessary to state the direction of the error, since if all the verniers 

 be read, the means of the readings in two different positions of the 

 instrument determine without error the angles through which the 

 circles have been turned from the one position to the other. 



The value of a scale-division of the striding- level is 1"12, that 

 of a scale-division of the attached-level is 2"*54. 



In the focus of the object-glass is a frame containing one hori- 

 zontal and five vertical threads. The intervals of the vertical 

 threads were determined by collimating each in succession on the 

 image of the fixed cross-wires of an opposed auxiliary telescope, 

 noting for each coincidence the readings of one of the verniers of 

 the horizontal circle of the instrument. The intervals being small, 

 it was unnecessary to read both verniers. In succession from the 

 apparent left of the field of view, i.e. from the side nearest the 

 revolving cone of the instrument, the intervals were 6' 40", 5' 40", 



17—2 



