106 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



4 inches. The tube of the telescope is in two parts, each 

 of which is attached to a central cube. Their diameters 

 decrease from 8.1 inches at the cube to 6.5 inches near 

 their outer ends. An eyepiece giving a power of 90 and a 

 field of 12' was used for these observations. The star 

 images formed are not exactly round, but are slightly elon- 

 gated in a direction parallel to the horizontal (declination) 

 thread. There being no component of this elongation 

 parallel to the vertical threads, it can have no effect upon 

 observations for zenith distance. 



The axis is 3 feet 2^ inches long, the distance between 

 the counterpoises being 2 feet 2 inches. The pivots are 

 3.6 inches in diameter and are protected by brass covers. 

 The telescope is furnished with clamps which, however, 

 were never used during these observations. After the 

 telescope was once set for a star it was not moved again to 

 make the bisection, this being done by means of the 

 declination micrometer. The value of one revolution of 

 the screw of this micrometer is 48". 10. This value has 

 been adopted as the result of many determinations made in 

 past years. The micrometer thread is single. 



The instrument has two circles, one of which can be 

 rotated about the axis of the instrument while the other is 

 rigidly fixed to it. They are both graduated to 2'. The 

 degrees, as numbered, increase counter-clockwise. The 

 diameter of the silver circle, upon which the graduations 

 are marked, is 26 inches. There are 130 graduations to 

 the inch. The fixed circle was used throughout these 

 observations. 



The four reading microscopes on each side are alike in 

 all respects. They are 26.5 inches long and have clear 

 apertures of 0.55 of an inch. Their powers are 40 and their 

 fields are nearly one degree. The objectives are 5 inches 

 from the circle and their eye ends project 8 inches from 

 the frame holding them. The micrometer heads are divided 

 into 60 parts. One revolution of a micrometer head carries 

 the threads over one minute of arc of the circle. There 



