20 THE MODERN REFLECTING TELESCOPE. 



diameter. For larger mirrors it is more economical, in the opinion of the writer, to 

 use half-size tools for obtaining a fully polished spherical surface, and the same and 

 smaller tools for parabolizing. The method of using these will be described 

 later. 



In genei-al, it is much easier to prevent the formation of zones, and to eliminate 

 zones already present, with full-size polishing tools than with smaller ones. The 

 method of manual polishing just described, in which the length of stroke and the 

 amount of side-throw are very frequently changed, tends to give a spherical surface, 

 except for a zone around the edge of the miiTor one-half an inch or less in width ; 

 this part of the surface will be of too great focal length, i. e., will turn down or 

 back slightly, unless means are taken to prevent it. This tendency is most pro- 

 nounced when a long stroke is used to excess, or when the rosin squares are too 

 soft. It is entii'ely prevented by diminishing the area of the rosin squares around 

 the edge of the tool, by trimming their edges to such a form as is shown in Fig. 4, 

 page 28. The exact amount of trimming required depends upon the length of 

 stroke, hardness of rosin, and tempei'ature of [)olishing room, and therefore can be 

 exactly determined only by experience. 



A 24-inch mirror which has been propei'ly fine-ground with emei'ies down 

 to 2-hour or 4-hour washed, is I'eadily brought to a perfect polish with a full-size 

 tool in from 2 to 4 houis of actual polishing. If several broad zones of different 

 focal lengths have resulted from the fine-grinding, as frequently happens, these 

 zones can be giadually eliminated by a continuation of the use of the full-size 

 polisher as above described. 



Attention must be given to the rosin squares, which gradually press down so 

 that their edges must be trimmed to keep the grooves of their original width and 

 of uniform width. When the bare rosin begins to show at the corners or edges of 

 the faces of the squares, which will occur after 6 or 8 hours' use of the tool, 

 a new coat of wax must be applied, and the tool must again be thoroughly cold- 

 pressed. It must not be supposed, however, that cold-pressing is necessary only at 

 such times ; in all fine work this pressing must be done whenever the tool has 

 remained off the glass for more than a few minutes ; after hanging face down during 

 the night the tool is always cold-[)ressed foi- about 2 hours before polishing is begun 

 in the morning. 



Polishing with half size oi- smaller tools is best done with the machine, instead 

 of by manual work. These tools do not have to be removed fr'om the glass in order 

 to renew the supply of loiige ; they aie therefoi-e connected to the machine and used 

 very much as half-size grinding tools aie used ; in my work they are made of such 

 weight that they need not be counterpoised. Very large or uiuisnally heavy polish- 

 ing tools of this kind can, of course, be easily counter[)oised when desired. 



Great experience, constant attention to very fretjuent changing of the position 

 of the tool by means of the transverse slide, and frequent testing of the form of the 

 mirror surface are necessary in polishing with half-size or smaller tools, in order to 

 preserve the uniform curvature of the surface. This is greatly facilitated by trim- 

 ming the rosin squares at and near the edges of the tool, as in the case of full-size 



