ASTRONOMICAL TELESCOPES — PEASE 207 



of the sj)ectrum, but which deviate or bend the rays by different 

 amounts. Two prisms of like material and angle can be placed so 

 as to produce a colorless beam, but they will have no power of 

 altering the initial direction of the beam. Two prisms of unlike 

 materials and different angles properly chosen will produce a nearly 

 colorless beam and still have the power of converging the rays to a 

 focus. By expanding these prisms into a convex and a concave lens 

 we obtain an achromatic lens, giving an image practically free from 

 color. 



The central image formed by a paraboloidal mirror is perfect, 

 but near the edge of the field of view the images are elongatetl 

 toward the center. This defect — coma, as the optician calls it-- 

 limits the size of the picture, but until further improvements are 

 made we shall continue to use the paraboloidal reliector for large 

 telescopes. 



To-day the telescope is moi-e «than the simple instrument through 

 which one looks at the heavens. The only way in which we can 

 make contact with the stars is to gather the radiations which they 

 send us and study the messages they bring. The lens or mirror 

 gathers these radiations and with them forms an image. This 

 image we examine not only with the eyepiece but with all the 

 modern auxiliary attachments such as plate holders, spectrographs, 

 photometers, and bolometers. With them we can learn many facts 

 about the stars, their chemical composition, sizes, masses, and den- 

 sities, their distances and shapes, their temperatures and their ages. 

 and how much longer they will probably live. 



Much of the work formerly done with the eyepiece is now accom- 

 plished by photography. The telescope is a huge camera, the tube 

 being the camera box. The eyepiece having been removed, the plate 

 is placed directly at the focus where the various parts of the image 

 affect it according to their brightness. Exposure times vary greatly ; 

 some for photometric work are of but a second's duration, while 

 those for a star cluster may continue for 8 or 10 hours. Photo- 

 graphs of the moon and planets take from a half second to several 

 seconds, and more if a color screen is used. One advantage that the 

 photographic plate has over the eye is the ability to accumulate the 

 effects of the light. Our pictures of nebulae accordingly show mar- 

 velous structures not visible in the eyepiece. 



The second far-reaching event in the history of astronomy took 

 place when Sir William Iluggins attached a spectroscope to his 

 telescope and viewed the spectrum of many stars. The telescope 

 alone taught us that there are other worlds than ours; the telescope 

 and the spectroscope together, that all tliese worlds are made of the 

 same materials as the earth. 



