[PLASKETT-DELURY] DETERMINATION OF THE SOLAR ROTATION 121 
There is fair agreement between the values of the rotation, although 
the two last are lower than they should be. This is probably due to 
the uneven illumination of the collimator and grating from the two 
limbs of the sun as the plates were made before the best methods of 
adjustment had been developed, and before the necessity of the greatest 
care in adjustment had been recognized. 
The agreement between the values for the different lines is good, 
corresponding to a probable error of linear measurement of less than a 
micron. This, however, was only attained by the greatest care and 
after considerable experience in measuring these rotation plates. 
It gives us great pleasure to acknowledge the readiness of the 
Director, Dr. W. F. King, to supply all the needed apparatus, and the 
encouragement he has given in the prosecution of the work. 
Dominion Observatory, 
Ottawa, 
June, 1911. 
