224 Praesepe Group; Measurement and Reduction 



scale under consideration and for the Rutherfurd plates, a cor- 

 rection for change in the scale- value will be unnecessary so long as 

 the temperature does not vary more than io° during the measure- 

 ment of a single plate. 



6° Projection Errors and Deviation of the Cylinder from 

 Straightness. 



In the present work corrections have been applied for neither 

 of these ; the first were discussed by Donner * and are very small 

 in most cases ; it is indeed very difficult to determine them with 

 sufficient accuracy. Repsold has recently devised a new guiding 

 way which is free from this source of error ; the measuring ma- 

 chine used for the Praesepe plates had been furnished with such 

 a guiding way in 1896. As regards the straightness of the cylinder, 

 investigation showed that it had been admirably made f ; the 

 greatest error which we shall commit in assuming it to be straight 

 is o. /; 04 ; the reversal of each plate and the insertion of different 

 plates in different positions in the measuring machine will tend 

 to eliminate even this small error. 



Having now completed the consideration of all the instrumental 

 corrections which it is necessary to apply, I shall conclude this 

 subject by correcting the measurements of Star 7, Plate Till, as 

 given on page 199, or in Table III. 



March 25, March 24, March 18, March 22, 



x direct : y direct : x reversed : y reversed : 



Lines 74,76 36 43,44 83 



2?n; First obs'v'r., . . 0.4646 — 0.0154 °-593S 0.5680 



Second " . . 0.4639 — 0.0178 0.5896 0.5670 



Mean, 0.4643 — 0.0166 0.59x7 0.5675 



Cor. for Euns, etc., — 9 o — 11 — 11 



Div. Correction, . . . . + 50 + 2 4 + 26 + 50 



Corrected |»i., 0.4684 — 0.0142 0.5932 o 5714 



Measurement, 75-4684 35-9 8 58 44-°93 2 83-5714 



Where two lines have been used the division correction is the 

 mean of the division corrections for the separate lines ; the final 

 " measurement " is then obtained by adding the mean of the num 

 bers of the lines to the corrected ^ m. The corresponding meas- 

 urements for the central star 15 when similiarly corrected are : 



* " Determination des Constantes n6cessaires pour la Eeduction des 

 Cliches." Acta Societatis Scientiarum Fennicae, Vol. XXI. 



t "Permanence of the Eutherfurd Plates " by Harold Jacoby ; Annals of 

 the N. Y. Acad, of Sciences, Vol. IX, page 210. 



