The Amateur Telescopist's Handbook (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER II. TESTING THE OBJECT GLASS.?EYEPIECES.?TUBES. The figuring of a telescopic objective is one of the most difficult and delicate operations in the arts, especially when its diameter exceeds five or six inches. This being the case, it is to be expected that the making of the highest grade of object glasses should remain, as it does, in the hands of a few artists of the highest intelligence and skill. The manufacture of perfectly homogeneous glass for this purpose is also a difficult operation. The result of these facts is that object glasses of the first class must necessarily be expensive instruments. An estimate (although so rough as to be of little use) of the cost of a high-grade objective may be formed by a rule given by Professor Newcomb: multiply the cube of the aperture in inches by $t and also by $1.75; the products will give the minimum and maximum respectively of the probable price. A first-class three-inch glass, according to this rule, would cost from 27 to 47.25; a four-inch from $64 to $112, and so on. I will give, however, in another place, the actual prices of object glasses as supplied by some of the best opticians of England and the United States. In testing an objective it should be first carefully examined in its cell. It should appear brightly polished On this subject see a most interesting article by Professor New- comb, " The Story of a Telescope," in Soibner'i Monthly for 1873, vol. vii. p. 44. and free from scratches, and preferably without bubbles or sand-holes. These things, it is truly said, only cause the loss of a little light, and are not incompatible with satisfactory definition; but the possessor of a small telescope can ill afford to lose any light. Still, these faults are not sufficient by themselves to warrant the ...