ASTRONOMICAL TELESCOPES ^ 



By F. G. Pease 



[With sevcu plates] 



Whenever one looks at any object, the crystalline lens in the eye 

 forms an image of that object on the retina. To see a distant object 

 more plainly and bring it apparently closer, it is necessary to en- 

 large this image. The instrument which does this is called a tele- 

 scope. It consists of two optical parts and a mechanical rod or 

 tube to hold them at the proper distance apart. One of the optical 

 parts, which may be either a lens or a mirror, forms an image near 

 the eye, and the other, called the eyepiece, magnifies it. A tele- 

 scope in which a lens forms the image is called a refractor; one in 

 which a mirror forms the image, a reflector. The size of the image 

 is determined by the focal length of the lens or mirror, and the 

 brightness of the image by its area. 



The first telescope of definite record was made by Lippershey in 

 Holland when he j:)laced a spectacle lens at each end of a tube, and 

 made terrestrial observations by holding it in the hands or by rest- 

 ing it upon a stand to steady it. Astronomical observations had, of 

 course, been made long before, at first with the naked eye, then with 

 simple measuring sticks held at arm's length, to be followed by aper- 

 tures cut in walls, or markings placed on ramps to observe certain 

 stars. As time passed, the use of graduated circles became common 

 practice, and just preceding the advent of the telescope came the 

 type of quadrant described by Hevelius and shown in Plate 2. The 

 observer sighted the star through the two markers upon the movable 

 bar which he adjusted up and down, at the same time rotating the 

 whole quadrant about the vertical post. All that could be done with 

 these instruments was to chart the relative positions of the stars and 

 planets with respect to one another. 



Upon learning of Lippershey's "optic tube," Galileo constructed 

 such a telescope of two spectacle lenses and a paper tube, and from 



1 A lecture delivered on Dec. 1.5, 1927, at the Publia Library, Los Angeles, under the 

 auspices of the Astronomical Society of the Pacific. Reprinted by permission, with 

 additional illustrations, from tlie Publications of the Astiononiical Society of the Pacific, 

 Vol. XL, No. 233, February, 1928. 



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