115 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



In the present or fourth epoch of astronomical progress, extraor- 

 dinary aids to observation have come. These include the applica- 

 tion to telescopes of great engineering construction and highly accu- 

 rate mechanisms, fully equipped with electrical devices for their 

 operation. The photographic plate, the thermionic amplifier, and 

 the sensitive thermoelectric element have nearly displaced the eye 

 at the observing end of the gigantic reflecting telescopes of the pres- 

 ent day. Photographic records in which light builds up its story 

 through many hours or even many nights may now be studied at 

 leisure in the comfort of an office, instead of glimpsed in an instant 

 at the eyepiece of a telescope in frigid winter midnight. Observa- 

 tions with powerful spectroscopes reveal the composition, motions, 

 distances, and temperatures of the stars. Based on the science of 

 thermodynamics, twentieth-century physical knowledge and studies 

 of the mysteries of radium and radioactivity, of the inner construc- 

 tion of atoms, and of the effects of powerful excitations of atoms 

 by electricit}' or heat, have settled questions of the natures of stars 

 that formerly seemed insoluble. Sucli studies have even disclosed 

 stars whose material is many times as dense as gold or platinum. 

 And yet more paradoxical, such stars as these are nevertheless in the 

 gaseous state ! On the other extreme are disclosed stars several 

 hundred million miles in diameter whose density is thousands of 

 times less than that of air. Sir William Herschel's happy guess that 

 the nebulae are other galaxies outside our own system, or, as he 

 called them, "island universes", has proved to be correct. Millions 

 of these remote galaxies have been disclosed, each like our own 

 Milky Way containing multitudes of stars, but situated so enor- 

 mously remotely that their light, traveling nearly 200,000 miles each 

 second, requires many millions of years to come from them to us. 

 We see them, therefore, as they were in former epochs of the earth's 

 geology, when gigantic creatures now extinct abounded. What food 

 would this have been to an imagination like our Shakespeare's ! 



Turning from these inspiring views of modern astronomical knowl- 

 edge, let us now explore more particularly some of the instruments, 

 the theories, and the observations which comprised the astronomy as 

 generally known in Shakespeare's time. As we have remarked, the 

 unaided eye had neither telescope, photographic plate, nor sensitive 

 radiometer to assist it. The positions of the heavenly bodies could 

 only be observed by casting shadows or by looking through sights 

 analagous to those of a sporting gun. These two types of naked 

 visual observation are exemplified in the sundial and in the astrolabe. 



Nevertheless, much information was gathered by the ancients 

 merely from the shadow of the sun. This laiowledge reached far 

 beyond its well-known use in the sundial to indicate the time of day. 



