18 THE MODERN REFLECTING TELESCOPE. 



The same question ai'ises in regai'd to the size of polishing tools as in tlie case 

 of giinding tools, — whether they shall be full-size or sniallei'. In the writer's o[)in- 

 ion fine plane and spherical surfaces up to about 36 inches in diameter are best 

 polished with full-size tools, which are moved by hand, by the optician and one or 

 two assistants, upon the surface of the slowly rotating glass. Tlie iippei- parts of 

 the machine are, of coui'se, removed during such polishing, which I shall call manual 

 polishing. 



A 24-inch polishing tool, pi-epai'ed as already described, with its wooden basis 

 2| inches thick, weighs about 25 pounds ; this is not heavy enough for the best ac- 

 tion in polishing ; so about 50 % additional weight is put on in the foim of 12 lead 

 blocks which are distributed uniformly and screwed to the back of the tool. 

 This gives a weight of about yV pound foi' each square inch of area, which is found 

 to work well for all large tools. For tools 18 inches or less in diameter somewhat 

 gi'eater pressure per squai'e inch of area may be used. A 36-ineh tool, with wooden 

 basis 3f oi- 4 inches thick, weighs 75 oi- 80 pounds, and needs no additional 

 weighting. 



The work of polishing a 24-inch miiror with full-size tool will now be described. 

 Six strong knobs of oak wood are screwed to the back of the wooden basis, each 

 knob being at the center of weight of each sixty-degi'ee sector of the tool. These 

 knobs serve for pushing, [)ulling, and lifting. 



The polishing tool, which, with the glass, should have cooled over night after 

 the warm-pressing or rough-pi'essing pi'eviously described, is now to be cold-pressed. 

 Cold-pressing is absolutely necessary in all fine woik on large optical suifaces. 

 In warm-pressing, both tool and glass are distorted by even slight warming, and 

 when they become cool a perfect fit cannot be expected. The glass is carefully 

 wiped with clean cheese-cloth, and an abundance of very thin mixture of I'ouge 

 and water is spread upon it. The tool is now placed upon the glass and allowed 

 to lie for several hours, being moved about slightly every ten minutes to i-edis- 

 tribute the rouge and watei", and to pi'eveut the latter from diyiug around the 

 edges. The pressing may be assisted at fii'st by means of a 20- or 30-pound weight, 

 the pressure of which must be distributed by some such means as three bai's 

 laid upon the six knobs, and a triangle, carrying the weight, laid upon these. The 

 final cold-pi'essing must be done by the weight of the tool alone. The tool is 

 taken off and examined occasionally ; when it is sufficiently pressed the wax sur- 

 face appears uniformly smooth and bright. So perfect a fit is secuied in this way 

 that there is no danger of injuring the form of the glass when polishing is begun. 

 This applies to all stages of polishing and figuring. A fresh supply of I'ouge and 

 water is now spread upon the glass. 



The stroke of the 24-inch polishing tool is easily given by the optician and one 

 assistant, who sit on opposite sides of the machine; the glass slowly I'otates with 

 the tuiiitable, making about 2 revolutions per minute. The knobs on the back 

 of the tool are held in the hands, and the stroke is given by alternately pushing 

 and pulling ; no vertical pressure whatever should be given by the hands. In 

 addition, a considerable side-throw is always given, first to one side, then to 



