HALL, ABERRATION CONSTANT. 



41 



of light falling on the mirror is regulated by opening or closing slats 

 placed in the axis of the telescope. 



As the Repsolds had only the old Pistor and Martins micrometer to 

 work from they declined to attempt any arrangement for lighting the 

 wires, for fear, I suppose, of producing an unsymmetrical illumination. 



For all observations the same eye-piece has been used, magnifying 160 

 times. This is about as high a power as it is possible to employ over the 

 nadir basin. A number of attempts were made to use higher powers, but 

 the nadir could not be obtained with them except when the air was quite 

 steadv. 



l.—TJiG Pivots. 



The pivots are approximately 1.819 inches in diameter. They must have 

 been very carefully made. Professor Schaeberle cleaned them, T under- 

 stand, with graphite and oil before beginning work with this instrument. 

 They were polished afterwards, also, by Professor Asaph Hall, in 1894, 

 with fine pumice stone and watch oil. The rust which had formed on 

 them had made etchings apparently, but had not injured their form. I 

 think they are as round now as when they were made. 



It was not possible to test the pivot inequalities with the hanging 

 level, since the wye blocks are so large that the level wyes cannot be placed 

 over the bearings of the pivots. To examine the pivots, therefore, I had 

 the observatory purchase from Saegmtiller a spherometer caliper. A de- 

 scription of this is given in Doolittle's Practical Astronomy. Though 

 exceedingly delicate measures can be made with it the results are not 

 very definite, as it is now constructed, for the upper and lower bearings 

 are not in the same vertical plane. It merely shows in a general way 

 whether or not the pivots are good. 



Before taking the measures the two telescope ends and the circles were 

 removed from the cube, and it was placed on a wooden support like that 

 of the reversing carriage. Settings of the caliper were made on those 

 parts of the pivots on which the wyes bear, and on which the level wyes 

 are usually placed. Eight positions of the cube were taken : eye-end up 

 and down ; edge of cube near 1854 in vertical, up and down ; side inscribed 

 Berlin, horizontal, two positions; edge near Pistor and Martins, in verti- 

 cal, up and down. These positions were 90° apart. 



Twenty settings of the caliper were made in each position. From i/o inch 

 and 14 i^ch steel blocks furnished by Brown and Sharp, one revolution 

 of the spherometer screw was found to be 



0.00974754 in. 



From the settings on the pivots we have the following results : 



BEARINGS ON Y'S. 



