286 



PROCEEDINGS OP THE AMERICAN ACADEMY 



the deviation of the optical axis of the telescope from this plane 

 by reading the circle, the level, aud the index of the micrometer 

 screw. In this way a constant watch may be kept ujjou the position 

 of the microscopes with respect to the circle. The whole apparatus 

 may be conveniently attached to the cube of the telescope. The 

 steadiness of both the level and the microscopes may be inferred 

 from the following readings of the circle for the zero line of the level. 

 They involve the accidental errors of reading both of the microscopes 

 and of the level, as well as actual changes in the position of the mean 

 of the four microscopes. That part of the change which is due to the 

 latter cause will be determined when the observations of Polaris are 

 reduced. Only the seconds of arc are given. 



