6 THE MODERN REFLECTESTG TELESCOPE. 



CHAPTER lY. 

 GRINDING TOOLS. 



Whii<e grinding tools of glass were used in mncli of my earlier woi-k, and are 

 still used for small work, I now use cast-iron gi-inding tools for all large work. 

 These aie cast very heavy, with ribs on the back ; the ribs are made heavy, but not 

 deep (or high). For large work iron tools are cheaper than glass ones ; they are 

 more easily pi'epared ; they are more easily and safely counterpoised, which is 

 always necessary in the fiue-gi'inding of large woi'k ; and they produce on the glass 

 a fine-ground surface fully as smooth and perfect as can be obtained with glass tools. 

 An important question is in regard to the size of grinding tools, — whether they 

 should be of the same diameter as the mirror. For mirrors up to 24 or 30 inches 

 in diameter full-size tools are generally used. For concave mirrors larger than 30 

 inches in diameter I use grinding tools whose diameter is slightly more than half 

 that of the glass, i. e., a 16-inch tool for a 30-inch glass; a 32-inch tool for a 60-inch 

 "•lass. These I shall refer to as half-size tools. Full-size tools ai'e, of course, much 

 more expensive and difficult to make; they are many times heavier than half-size 

 tools of equal stiffness; and they require a much stronger grinding machine to 

 counterpoise them properly; grinding can be done with them, however, more 

 quickly than with the smaller tools. Half-size tools are economical and are quickly 

 prepared ; they are easily counterpoised ; and a much greater vai-iety of stroke can 

 be used with them, so that with a well-designed grinding machine I have found it 

 easier to produce fine-ground surfaces, entirely free from zones, with half-size than 

 with full-size tools. If temperature conditions and uniform rotation of the glass 

 are carefully attended to, the surface of revolution produced by the smaller tools is 

 fully as perfect as that given by the larger ones; I always take the precaution, 

 however, to work a fidl-size approximately flat tool on the glass befoi-e beginning 

 to excavate the concave, so as to start out with a surface of revolution. 



Grinding tools for concave and convex mirrors are always made in pairs, one 

 concave, the other convex. Grinding tools for plane mirrors are made in triplicate. 

 These iron tools, when being cast, are "poured" face down, so that the faces will 

 be clean. I shall describe the preparation of a pair of iron tools for a concave 

 mirror, leaving the description of tools for plane mirrors until the making of jilane 

 mirrors is discussed. The convex and concave tools are turned in a lathe to the 

 proper curvature as shown by templets. The convex tool, which is, of course, to 

 be used on the concave glass, is now placed on a planing machine, and has a series 

 of grooves cut across the convex surface. These grooves are usually \ inch wide, 

 and run in two directions at right angles to each other; these divide the surface into 

 squares, which are usually made about one inch on a side. These grooves serve to 

 distribute the grinding material uniformly, and entirely obviate the tendency of the 

 tools to clino; to the o;lass in fine-gi-inding. No grooves are cut in the concave 

 tool. A number of holes are now bored through both tools, in such positions that 

 wooden cups or funnels can be inserted into the holes from the back or i-ibbed side 



