144 Transactions of the Society. 



VI. — On the Measurement of the First Nine Groups of Grayson's 



Finest Twelve-band Plate. 



By A. A. C. Eliot Merlin. 



(Bead February 16, 1910.) 



I have recently effected the measurement of the first nine groups 

 of Grayson's finest plate containing twelve bands of rulings, com- 

 mencing at yo&oo m - s P aces > an( l increasing up to 120W0 * n - ^o 

 far as I am aware, no attempt has hitherto been made to span such 

 close lines with the screw micrometer wire, and it has consequently 

 been thought that the outcome may prove worthy of record. 



The measurements, details of which are contained in the 

 annexed table, were effected by means of a y 1 ^ Powell oil-immer- 

 sion objective of measured N. A. 1 " 27, a negative amplifying lens, 

 increasing the initial magnification of the objective about two and 

 a half times, and an ordinary Powell screw micrometer furnished 

 with a 6 eye-piece. With this optical combination, 134*5 drum 

 divisions were found to equal yoooo * n "' ^ nus ma k m » a movement 

 of the wire through one division equal to 7345(500 m - ^ * s nere > 

 perhaps, hardly necessary to remark that in the highly accurate 

 determination of intervals, well within the defining power of a 

 lens, the separating limit does not enter as a factor. 



In order to eliminate and indicate the micrometer screw error, 

 and to make evident the true relative accuracy of these beautiful 

 rulings, in addition to the values of the individual line spaces 

 given in the first of the two columns devoted to groups 2, 3, 4, 5, 

 6, and 7 in the annexed table, which were necessarily effected 

 with varying and sometimes wholly different portions of the screw, 

 a second column exhibits the readings obtained by spanning two 

 spaces throughout the second band, three throughout the third, 

 and so on up to seven throughout the seventh group, utilising for 

 all the spaces the exact portion of the screw used for each of the 

 TUooo i n - li nes °f the first band, so that the means of the five 

 resultant readings, given in these columns, are free from micro- 

 meter screw error, and indicate the true value of the various 

 groups in terms of the first. 



The first seven bands ( To -ooo ^° 70000 m -) were measured 

 under strictly critical conditions with the full axial illuminating 

 cone of Powell's dry apochromatic condenser, affording a working 

 aperture of N. A. 0'95, or thereabouts, used in conjunction with 

 Gifford's F-line screen. For the eighth and ninth bands (5000U 



