THE MODERN REFLECTING TELESCOPE. 7 



of the tool, without interfering with the ribs; these cups serve for the introduction 

 of the grinding material during the process of grinding; they should be thoroughly 

 vainished. 



The convex and concave tools are now ground together on the machine, with 

 fine grades of carborundum (which is much more effective for this purpose than 

 emery) and water. This eliminates the circular marks left by the lathe, and 

 enables the optician to secure the exact curvature desired. A very important point 

 is that by grinding with the concave tool on top, the radii of curvature of botii 

 tools can be gradually shortened ; when the convex tool is used on to[) the curva- 

 ture of both is gradually flattened. By this means, and the use of very fine grades 

 of carl)orundum, a most perfect control of the curvature of the tools may be had. 



The curvatui'e of the tools and of the glass is measured by means of a large 

 spherometer; this is shown in Plate v, resting upon a 12-inch glass grinding tool. 

 The spherometer is of the usual thi'ee-leg form ; the legs terminate in knife-edges, 

 the lines of which are parts of the circumference of a 10-inch circle. The central 

 screw is very carefully made ; it was ground in its long nut (which was made adjust- 

 able for tightness) with very fine grades of emeiy such as are used in optical work ; 

 screw and nut were then smoothed and polished by working them together with 

 rouge and oil. The screw is of ^ millimeter pitch, and the head, which is 4 inches 

 in diameter, is graduated to 400 divisions. On fine-ground surfaces settings can 

 be made to one-half or one-third of a division, corresponding to a depth of -^^qq or 

 TTTroUTr ^^ ^° inch, appi'oximately. 



CHAPTER V. 

 POLISHING TOOLS. 



After experience with polishing tools of various kinds, the tools which I now 

 use exclusively for lai-ge work consist of a wooden disk or basis constnicted in a 

 peculiar mannei', and covered on one side with squares of rosin faced with a thin 

 layer of beeswax. The wooden disk may be replaced, in the case of small polish- 

 ing tools up to 12 or 1.5 inches diameter, by a ribbed cast-iron plate so designed as 

 to be extremely light and rigid ; the bases of lai-ger tools may be made of cast 

 aluminum, but this, in order to be strong and rigid, must contain 15 fo or more of 

 other metals; sucli a basis for a 30-iucli polishing tool weighs about sixty pounds, 

 and the rough casting ah>ne costs about fifty dollars. It is possible that a metal 

 basis possesses an advantage over a wooden one in that its surface is less yielding. 

 Tools properly constructed of wood, however, are light and extremely I'igid, are 

 easily made, and ai-e economical in cost. As their proper construction is a matter 

 of the utmost im[)ortance, I shall desciibe, somewhat in detail, the method of 

 making; wooden bases of from 15 to 40 inches diametei-. 



A large number of strips of dry and straight-gi'ained pine wood 1 J inches wide 

 and -^ inch thick are prepared; the wooden basis is built up of successive layers 



