THE MODERN REFLECTING TELESCOPE. 17 



focal length of the finished paraboloid and R is the semi-diameter of the miri-or ; 

 the reason for this is fully explained later. I have iievei' attempted to parabolize 

 while fine-grinding; it is possible that it miglit be well to do this in the case of 

 very large mirrors of shoi't focus, but my practice has been to fine-grind and polish 

 to a spherical surface, free from zones, and then to parabolize by means of suitable 

 polishing tools. 



CHAPTER X. 

 POLISH IN(i. 



The prepai'ation of polishing tools has already been described. The polishing 

 rouge which I use is of the quality which is used in lai'ge quantities commercially 

 in polishing plate-glass. I prefer the powdered form always. This grade of rouge 

 is not expensive (it costs about 30 cents per pound), but, like all rouge which 

 I have seen, it contains hard, sharp particles which may cause scratches. It must 

 therefore be thoroughly washed in the following manner : 



In a clean, deep bowl C enough rouge is placed to fill it about one-third full ; 

 the bowl is then nearly filled with distilled water. The mass is very thoroughly 

 stirred with a clean wooden paddle, and allowed to settle for about twenty minutes. 

 The water above the i-ouge will now be perfectly clear ; this water is siphoned off. 

 With a clean spoon the light and fine rouge constituting the upper one-third of the 

 precipitated mass is I'emoved, and placed in a second clean bowl D. The rouge 

 remaining iu C'may be again stirred up with an abundance of distilled water, and 

 allowed to settle as before, the water siphoned off, and the upper one-fourth of the 

 precipitated rouge removed and placed in D. The heavier rouge which remains 

 in G is about half of the original quantity taken ; this is usually reserved, and, 

 after further washing, is used for polishing the backs of mirrors, and for similar 

 work. Only the contents of the bowl D ai-e used for fine work, and these are stiri-ed 

 up again and again with distilled water dui'ing the process of polishing, and onlj'' 

 the fine, soft ci'eam which remains on the top of the mass of rouge, when it settles 

 each time, is used for polishing. 



The thin cream of I'ouge and distilled water is applied to the glass by means of 

 a wide brush consisting of a thin paddle of wood with clean cheese-cloth wrapped 

 and tied about one end. Brushes of the usual kind should not be used. 



By taking these precautions, and by the use of the wax surface on the rosin 

 squares, scratches in polishing can be entirely avoided. It is true that the very 

 light, fine rouge polishes more slowly than the heavier and coarser rouge, l)ut an 

 exquisitely fine polished surface is produced on the glass b}' its use. The wax 

 surface also polishes moi-e slowly than a baie I'osin one, but it has the very great 

 advantage that its action is more smooth and uniform than that of the rosin surface ; 

 tiie latter often tends to cling to the glass, and this unequally in different parts of 

 the stroke. 



