i 4 SCIENCE PROGRESS 



were secured, 2,100 were measured, and more than 200 paral- 

 laxes were determined. Results for 81 stars are in course of 

 publication. At the Dearborn Observatory, 800 plates were 

 obtained and 2,600 plates are awaiting measurement, which 

 was delayed pending delivery of a new measuring machine. 

 Results for 17 stars have already been received. At the 

 Leander McCormick Observatory, 4,200 plates have been 

 taken. Preliminary results for 83 stars have been published 

 {Pop. Astr. 25, 23, 191 7) and a complete account of the work 

 is being prepared. At Mount Wilson the 60-inch reflector is 

 being used for parallax work : this is the only observatory 

 not using a refractor. The telescope has the long focal length 

 of 80 feet, and this has enabled results of a high accuracy to 

 be obtained, the probable errors averaging only about o**oo6. 

 The programme includes many stars of small proper motion 

 or of advanced types and a number of planetary and other 

 nebulae. Two series of results have been published, of 20 

 stars in Mt. Wilson Contr. No. 3, and of 30 in Mt. Wilson 

 Contr. No. 136. The programme at the Sproul Observatory 

 comprises mostly visual and spectroscopic binaries. A first 

 list of 50 parallaxes was published recently in Sproul Observa- 

 tory Publications, No. 4, and a number of other results will 

 shortly be available. At the Yerkes Observatory, the 40- 

 inch refractor is being used regularly for parallax determina- 

 tions, and the results obtained to date are given in the Publica- 

 tions of the Yerkes Observatory , vol. iv. Pt. i. Some of the 

 results had previously been published elsewhere, but all are 

 collected for convenience of reference in this volume, which 

 contains results for 131 stars. At Greenwich, the parallax 

 observations have had to be temporarily suspended, but after 

 the war probably at least 40 results yearly will be obtained. 

 The programmes at the various observatories overlap by 

 design to a certain extent. This is necessary in order that 

 the results may be intercompared and a check obtained on 

 the existence of systematic errors, which are more to be feared 

 than accidental errors. The scheme of co-operation eliminates, 

 however, unnecessary overlapping. 



Reference must also be made to the spectroscopic method 

 of determining parallaxes developed by Adams. The principle 

 of this method has been explained in these notes (Science 

 Progress, 11, 97, 19 16). The first-fruits of it are now being 



