MOUNT WILSON OBSERVATORY. 215 



SECONDARY STANDARDS OF WAVE-LENGTH. 



Miss Margherita Burns and Miss Brayton, acting under the direc- 

 tion of Mr. Babcock, have continued the determination of secondary- 

 standards of wave-length in the iron spectrum from photographs made 

 by him in 1917. 



Since the last annual report they have separately completed the 

 measurement and reduction of 35 plates and the reductions of 30 

 plates previously measured. This brings the total up to 79 plates, 

 nearly all of which have been measured and reduced by two observers 

 independently. The remarkable accordance of their measures is demon- 

 strated by the fact that 25 fairly representative lines belonging to the 

 Hst of adopted secondary standards, including some of low weight, 

 show a mean difference, wdthout regard to sign, of only 0.0005 A. 

 Between the limits X 3370 and X 5662 we now have wave-lengths, 

 obtained by the interference method, of 836 iron lines, a large propor- 

 tion of which have been measured on more than 10 plates. The num- 

 erous new secondary standards thus made available are of considerable 

 advantage in the concurrent determinations of wave-lengths by means 

 of the plane grating, now proceeding under the direction of Mr. St. 

 John. The accordance in the results attained by these two methods 

 is so far very satisfactory, and it will doubtless eventually insure a 

 thoroughly reUable system of standards. 



CONSTRUCTION DIVISION. 

 DRAFTING AND DESIGN. 



The greater part of the time of the drafting department has been 

 given to the 100-inch telescope. The more important drawings and 

 designs relating to this instrument and its accessories which have 

 been prepared during the year include the mechanism for operating 

 the louvres in the dome, the Cassegrain spectrograph, modified mount- 

 ings for the coude and Cassegrain mirrors, a revision of the design of 

 the observing platform, the reinforcing ring for the Cassegrain cage, 

 and a complete sectional assembly through the telescope and dome 

 to be used for pubHcation purposes. 



Much time was also spent in the design and di'afting for special 

 apparatus used by the eclipse expedition, comprising three spectro- 

 graphs and a photoheliograph. 



The miscellaneous work has included a prism attachment for the 

 13-foot spectroheHograph, a new focal-plane spectrograph for the 

 60-inch reflector, attachments for the ruling engine, a solar stereoscope, 

 a laboratory grating spectrograph, various assembly drawings, and 

 numerous charts, etc., for the illustration of publications. 



During the absence of Mr. Pease in Washington, the department 

 has been in charge of Mr. Nichols, chief draftsman. 



