INSTRUMENTAL. DEVELOPMENT HALE 191 



stars, has recently provided the means of detecting the last wave 

 lengths missing in the long range from the gamma rays of radium to 

 radio waves 20,000 meters in length. The interferometer, springing 

 from Young's famous interference experiment of more than a century 

 ago, has served for scores of brilliant successes, recently culminating 

 in the determination of the angular diameters of giant stars. 



Without attempting to enumerate more of the astronomer's long 

 list of debts to the physicist and chemist, let us look for a moment at 

 the increase in the precision of measurement effected by instrumental 

 advances. The star places of the Greek were given to the nearest 10' 

 of arc, one-third the diameter of the moon. Tycho succeeded in 

 reducing the probable error of a single measure of the distance be- 

 tween two neighboring stars to 57". In double star observations the 

 probable errors of the best micrometric measures are about 0".l. In 

 modern photographic parallax determinations the probable error is 

 about 0".005 to 0".010. With the interferometer, the probable error 

 of a single measure of the separation of the components of Capella 

 is 0".001. The diameter of Arcturus, 0".019, can be similarly meas- 

 ured with a probable error of about the same amount. 



The advantages to be gained by the early utilization of the rapid 

 progress of the physicist and chemist are obvious. Almost any dis- 

 covery may help us directly or indirectly. We are interested in new 

 organic dyes because they may improve the sensitiveness of our plates 

 in various regions, especially in the infra-red, a most promising field 

 for future research. We earnestly hope for a reduction in the size of 

 the grain of the most rapid photographic plates, which would be 

 equivalent to a marked increase in the aperture of our telescopes. We 

 keenly watch for the appearance of new alloys, perhaps suitable for 

 telescope mirrors or for the special needs of optical gratings; pro- 

 gress in the manufacture of optical glass; the production of large 

 masses of clear fused quartz for prisms or mirrors — every technical 

 advance, in fact, that we can learn to utilize. And we are equally 

 anxious to benefit by the constant improvement of high-tension trans- 

 formers, electric furnaces, vacuum tubes, electromagnets, and the 

 many other devices on which we depend for the imitation and inter- 

 pretation of celestial phenomena. 



These illustrations of the increasing possibilities of instrumental 

 development have not been enumerated in strict chronological se- 

 quence, but a glance at this partial list will show how rapidly the 

 opportunities of the astronomer have multiplied in recent years. 

 Another point should be noted: The obvious chance is not always 

 the most important one, and the greatest advances may come from the 

 recognition of possibilities that are not immediately apparent. 

 Hence the astronomer can not watch too intently the progress of re- 

 lated sciences, and especially the numerous devices and methods 



