THE MODERN REFLECTING TELESCOPE. 19 



the other; this greatly assists in preventing the formation of zones of unequal cur- 

 vature. Polishing may be begun with a stroke 6 inches in length, which of course 

 causes the tool to overhang the glass 3 inches at the ends of the stroke; between 

 20 and 25 double strokes per minute are given. The side-throw used with this 

 length of stroke is about 2 inches, i. e., the tool is made to overhang the glass 

 about 2 inches, first to the right, then to the left ; the time occupied in passing from 

 the extreme right to the e.Ktreme left is about what is requii'ed for 4 double strokes. 

 This stroke and side-throw are continued while the glass makes exactly 2 revolu- 

 tions ; the tool does not rotate with the glass, of course, while the stroke is beiu<'' 

 given ; the last stroke should end with the tool central upon the glass. 



Tool and glass are now allowed to rotate together for | of a complete i-evolu- 

 tion, and each optician then grasps the pair of knobs next to that which he held 

 before, so that the stroke is now given along a diameter of the tool 60° from that 

 last used ; the length of stroke is now changed to 1 inches, and the side-throw 

 to 2i inches, and polishing is again carried on during exactly 2 revolutions of the 

 glass. Tool and ghiss are again allowed to I'otate together for -| of a revolution, and 

 polishing during 2 i-evolutions is now done with a stroke of 8 inches and side-throw 

 of 3 inches. During the next periods of polishing, each of 2 revolutions of the 

 glass, the stroke and side-throw are gi'adually shortened until a stroke of 4 inches 

 or less is reached; then the length of stroke is increased again. 



When polishing has been carried on during 6 oi' 8 pei'iods of 2 revolutions 

 each, it will be found necessary to supply more rouge. The only entirely satisfac- 

 tory method of doing this, when a full-size polishing tool is used, is to I'emove the 

 tool from the mirror, and quickly spread the thin cream of I'ouge and water upon 

 the glass as uniformly as possible with the cheese-cloth brush. The removal of 

 the tool is effected by the two opticians carefully sliding it off the mirror, and lift- 

 ing at the same time. The tool should be allowed to remain off the glass for only 

 as short a time as possible, so that the form of the latter shall not be altered as a 

 result of a change of tera[ierature of the surface, caused by evaporation. For this 

 and other i-easons, such as the prevention of dust, the air in the polishing room 

 should be kept moist by keeping the floor well sprinkled. 



When the tool is replaced on the mirror it is lifted by both opticians so that 

 only a very small part of its weight remains on the glass, and is lightly moved about, 

 for 30 seconds or more, to disti'ibute the I'ouge and water thoroughl}' l)efore polish- 

 ing is continued. As before stated, the method just described is the only entirely 

 satisfactory one, known to the writer, of supplying rouge during the polishing with 

 a full-size tool. All methods of sup])lying rouge at the edge, or through holes in 

 the tool, are inadmissible when the greatest refinement of figure is required. 



It is in order that they may be easily handled in the manner described that 

 full-size polishing tools should be made light. It would, of course, be possible to 

 devise mechanism by which tools of any size and weight could be sufficiently 

 counterpoised, could be moved about upon the glass, and could be removetl from 

 the latter for the purpose of supplying rouge. The sim[)le and economical method 

 which I have described, however, works well for mirrors up to 36 or 40 inches in 



