THE JIODERN REFLEOTrNG TELESCOPE. 



of these stiips. The strips iu all layers except the two outer ones are laid just J 

 of an inch apart. Those of each layer are placed at right angles to those of the 

 next layer below, and are glued and nailed down with long wire bi'ads. The best 

 cabinet-maker's glue is used, and the strips ai'e wai'nied before the glue is applied. 

 Each crossing of the strips in the successive layers (*'. e., each of the 1^-inch squares), 

 is nailed with at least two nails. The upper surface of each layer is carefully 

 planed flat before the next layer of stiips is apjilied. For a 20-inch tool six la^-ers 

 of pine stiips (eacli ^^-y iuch thick) are used ; foi- a 24-inch tool, seven layers; for a 

 36-inch tool, ten layers. Next, one layer of thoroughly seasoned strips of hard 

 straight-grained cherry wood about | inch tliick and slightly less than 1^ inches 

 wide is added, to form the outer layer at the back of the tool ; these strips are laid 

 almost touching each other. In the case of tools for flat mirroi's, a precisely similar 

 layer of cherry strips is added to form the outer layer at the front or face of the 

 tool. But in the case of tools for concave or convex mii'rors the strips composing 

 the front layer must be made thicker, to allow foi- the cui'vature of the face of the 

 tool. If this curvature is great, the cherry strips forming the front layer are inade 

 of double width (^. <?., slightly less than 3 inches wide), in order that the width of 

 their bases shall be greater as compared with their thickness; this is usually done 

 when the (le[)th of the curve is greater than | inch. The gluing and nailing of 

 the outer layers of stiips ai'e done with the greatest th(U-oughness, four of the long 

 fine nails being driven thi'ough into each of the squares of pine wood beneath. 

 For tools less than 20 inches in diameter thinner strips and a larger number of 

 layers are used. The entire thickness of the wooden disk or basis built up in this 

 way should be between one-tenth and one-eighth of its diameter. 



This wooden l)asis is next placed in a large lathe, the edge is turned smooth 

 and to the proper diameter, and the face is turned to fit the curvature of the glass 

 to be polished. 



A round pan of galvanized iron large enough to contain the wooden disk hav- 

 ing been prepared, enough hard ])araffin is melted in it so that the disk can be 

 soaked in the liquid paraffin ; the latter must not be hotter than 150° Fahrenheit, 

 otherwise the strength of the ghie-joints will be injured. It is best to melt the 

 paraffin on a gas or gasoline stove, so that the degi'ee of heat can be easily controlled. 

 The tool should soak for several hours, being moved continually and turned over 

 often. Since the construction of the wooden basis is such that a great number of 

 openings extend entirely through it, the melted paiaffin permeates the entire tool 

 thoroughly. The wooden tool prepai'ed in this way is lighter than any metal tool 

 of the same degree of stiffness, and is entirely impervious to the moisture which is 

 necessary in the polishing room. The question of lightness is a most important 

 one, as will be seen when the work of polishing is described later. 



The front or face of the wooden basis is now lightly scraped with a wide, 

 sharp chisel, to remove any excess of paraffin, and is then marked off for IJ-inch 

 squares of rosin, with grooves J iuch wide between them ; the grooves should come 

 exactly above the ^-inch spaces left between the pine strips beneath. 



The preparation of the rosin squares is usually a veiy troublesome matter, but 



