190 BIGELOW 



planes. The radii of curvature, measured with the sphero- 

 meter, were found to be 165. 7cm. for the outer curve, 274.4 ^^^■ 

 for the inner curve. The structure of the glass was examined 

 by means of Nicol prisms at conjugate foci. For perpendicular 

 position of the prisms the lens instead of being entirely dark 

 .shows irregular light portions extending toward the center, due 

 to irregular polarization in the glass. Practically, however, 

 the lens gives excellent star images for meridian circle work, 

 i. €., small, round disks, of uniform size across the entire field. 



The graduated circles of the instrument are 27 H iriches in 

 diameter. The fine circle, which was the one employed, is 

 graduated to 2' and is read by 4 microscopes of 16 magnifying 

 power reading to tenths of a second of arc. Each microscope 

 has been furnished with two sets of threads one and a half 

 revolutions apart to eliminate periodic error. For a reading 

 two divisions of the circle were pointed on, the micrometer 

 screw being turned always one half revolution. The readings 

 were corrected for error in the run. 



The micrometer eye-piece was obtained a few years ago 

 from the Repsolds. It contains 25 verticle threads in groups of 

 5, and 2 horizontal threads about 5" apart. There is no declin- 

 ation micrometer screw. Settings were made with the tangent 

 screw of the instrument, bringing the star to the point half-waj'- 

 between the horizontal threads. It was usually found possible 

 to make 3 or more pointings with the corresponding readings 

 of the microscopes while the star was crossing the field. The 

 positions off the meridian were symmetrically chosen to avoid 

 error caused by possible inclination of the wires. The reduc- 

 tions to the meridian were made according to the formula 



, . ^ V sin^ i^/ 



Z = Z' sin 2 O' — V yr 



sm 1" 



where o' is the apparent declination. In this form a second 

 term becomes negligible. (See Leyden Observations, Vol. VI, 

 p. LX.) Tables were made out for each star from which the 

 correction could be taken with the declination and hour angle 

 as arguments. 



Observations for nadir were made about every 3 hours. These 



