360 KANSAS ACADEMY OP SCIENCE. 



THE USE AND CARE OF REFLECTING TELESCOPES. 



By W. F. HoYT, Kansas Wesleyan University, Salina. 

 Read before the Academy, at Topeka, December 30, 1904. 



TN the precision of its adjustments, in the wealth of mechanical de- 

 -■- tails, in the number and intrinsic value of its attachments, the 

 modern telescope is the prince among human instruments. There 

 are two main kinds of telescopes, each with its fundamental advan- 

 tages and disadvantages. The power of an instrument is determined 

 largely by its light-gathering ability. Since the surface of a lens 

 varies as the square of its diameter, the greater the aperture, other 

 things being equal, the better the telescope. The refractor excels the 

 reflector somewhat in light-gathering efficiency, because a lens inter- 

 cepts less light than is lost by two reflections, as is necessary in mod- 

 ern reflectors. The small diagonal mirror, too, intercepts a consider- 

 able percentage of the light before it reaches the large speculum. 

 The definition of refracting telescopes is usually "superior, owing to 

 slight possible distortion of mirrors, or to the unevenness of the sil- 

 ver coating. Refracting telescopes are but slightly affected by time 

 or usage, while reflectors are tarnished after a few years, and must be 

 repolished or resilvered, or both. On the other hand, a good reflector 

 is absolutely achromatic, while chromatic aberration is inevitable in 

 the best refractors. The aberration of the violet and other actinic 

 rays in refractors render them undesirable for photographic work, for 

 which reflectors are especially adapted. Owing to this, every well- 

 equipped observatory has one or more reflectors as general working in- 

 struments. It was the two-foot reflector in the Yerkes observatory, for 

 instance, instead of the great forty-inch refractor, that made the fa- 

 mous discoveries concerning Nova Persei recently. Mr. Schaeberle, 

 of Ann Arbor, Mich., has constructed a large reflector with only an 

 eighteen-inch focus, with which he claims to have performed marvels 

 of photography, securing in a few seconds images of faint objects 

 which have heretofore required hours of exposure. 



We have probably reached the limit of size in refractors, but if we 

 may believe Mr. Ritchie, of the Yerkes observatory, we have not be- 

 gun to develop the possibilities of the reflector. He not only claims 

 that a ten-foot reflector is possible, but he offers to construct one if 

 funds are secured. Such an instrument would extend the area of the 

 visible universe a thousand fold. Cheapness of construction and ease 

 of management, however, are prime arguments in favor of reflecting 

 telescopes. A good modern reflector, of twelve-inch aperture, mounted 



