,9o6] PICKERING— INTERNATIONAL SOUTHERN TELESCOPE. 47 



been secured with any form of refractor. One great advantage of 

 the reflector is its low price. The cost of a mirror is about one 

 tenth of that of a lens of the same size. The great defect of large 

 refractors, the color of the images, due to chromatic aberration, is 

 not present in reflectors. The loss of light by absorption increases 

 rapidly with the size of the refractor, and not at all wath a reflector. 

 The difiference in focus of rays of different colors is so great with a 

 large refractor that the small dispersion needed for photographing 

 the spectra of faint stars cannot be used. No such difference exists 

 with a reflector. On the other hand, the reflector is much more 

 sensitive to changes of temperature or flexure, and the silver surface 

 becomes tarnished and must be renewed at intervals. Nevertheless, 

 in a very large instrument the advantages of a reflector far outweigh 

 those of a refractor. 



It is therefore proposed that the telescope shall be a reflector 

 having an aperture of about seven feet, and a focal length of forty- 

 four feet, thus giving images on a scale of 15" to the millimeter. 



Let us imagine the instrument completed, and describe its prob- 

 able construction. The polar axis is enclosed in an iron cylinder,, 

 resembling a boiler, and resting in water according to the method 

 adopted by Mr. Common in the construction of the sixty-inch reflec- 

 tor now at Cambridge. There will be no difficulty from freezing, 

 as the instrument will doubtless eventually be erected in a location 

 having a warm climate. It would be better, if possible, to counter- 

 poise the telescope, taking a large part of the weight off the bearings 

 of the polar axis by a series of ball bearings, if motion of suflicient 

 uniformity can thus be secured. Electric motors furnish abundant 

 power for the motions in right ascension and declination, and a 

 motor controlled by a clock is used for following. This method was 

 employed with entire success in the Harvard telescope, 135 feet long, 

 sent to Jamaica in 1901, and in other telescopes. (See Astrophysical 

 Journal, XV, 202.) 



The photographic plate is placed at the principal focus of the 

 telescope. For visual work, this is replaced by an inclined mirror 

 which reflects the beam of light to the side of the tube. It then 

 falls upon an astronomical objective of five or six inches aperture, 

 and after undergoing a second reflection, is brought to a focus in 



