C0M8T0CK — STUDIES IN ASTRONOMY 59 



thread. Then shift the aperture to the lowest part of the 

 objective and note whether the image of the star is sensibly 

 displaced from the thread. If the image moves in the same 

 direction with the aperture in the screen, the eye end 

 should be drawn out; if in the opposite direction it should 

 be pushed in until a position is found at which there is no 

 displacement of the star image. 



By this process the telescope may be so adjusted that 

 the error of focusing shall not exceed 1 : 10000 part of the 

 focal length, provided it is so firmly supported as to be 

 free from the eifect of accidental tremors and vibrations, 

 e. g. the telescope of a transit instrument. 



