of the Eulherfurd Photographs. 191 



with the telescope-tube. The last column, marked " focus," gives 

 the reading of a micrometer head attached to the eye end of the 

 telescope and shows the position of the plate holder ; this infor- 

 mation is not used in the reductions and is given only to provide 

 for the possibility of determining a relation between this reading 

 and the scale-value after a sufficient number of the photographs 

 made with this instrument has been reduced. 



