906 



Popular Science Monthly 



A group of women at the Harvard College Observatory, 

 whose duty it is to study the photographs and record them 



telescope an instrument is created which is 

 almost the exact optical equivalent of the 

 human eye. 



Thanks to this combination of the photo- 

 graphic plate and the telescope, the field 

 of astronomical exploration has been almost 

 immeasurably broadened. It is often 

 literally possible to discover things without 

 actually seeing them. 



Among the planets there is a group of 

 very tiny faint bodies which 

 are known as asteroids. Be- 

 cause of their very great 

 number, only the approxi- 

 mate orbits have been com- 

 puted for most of them. 

 This fact alone makes it 

 difficult to find them on an 

 ordinary star chart. Here, 

 the photographic plate has 

 proved itself to be of immense 

 value. 



Two photographic methods 

 of searching for asteroids are 

 in use — methods which are 

 the reverse of each other. 

 Both are illustrated in the 

 accompanying photographs. 

 The first, employed for many 

 years, is based on the fact 

 that asteroids can be detected 

 by their movements among 

 the stars. Ordinarily, when 

 a telescope is used for photo- 

 graphing the heavens, it is 

 connected with clockwork 

 which carries it just as fast 

 as the skies revolve, so that 

 the stars remain fixed in the 



field of view and appear on the photo- 

 graphic plate as round, sharp images. 

 If, on the other hand, there is present 

 a body which is moving among 

 the stars, it will produce a trail 

 on the plate instead of a round 

 dot. An asteroid is just such a 

 moving body. Therefore it 

 can be recognized by the trail 

 which it leaves and be made 

 an object of further study. 

 Many of the smaller ones, how- 

 ever, make trails which are too 

 faint to be detected in this way. 

 An ingenious method of over- 

 coming the difficulty has been 

 devised by Rev. Joel Metcalf, of 

 Winchester, Mass. He computed 

 in advance what the average 

 motion would be of an asteroid 

 in the particular part of the sky opposite to 

 the sun at any desired time, and gave to 

 his telescope a motion equal in amount but 

 opposite in direction. As a result, the 

 asteroid produced a round dot on the plate, 

 while the stars left trails. 



Discovering the Events of Yesterday 



Because the photographic plate is not 

 only extraordinarily sensitive, but also 



Early photographs of Halley's Comet which were taken 

 at the Yerkes Observatory in 1909. It appears on the 

 photographic plate as a star of the seventeenth magnitude 



