THE MODEKN REFLECTING TELESCOPE. 21 



tools, but to a gi'eatei- extent ; the effect of the action of the edges of the tool 

 is thus softened or blended. 



When a half-size or smaller tool has just been coated with wax, or is known 

 to be far from the exact foim desired, it is first cold-pressed in the usual way on the 

 center of the glass. But the final cold-pressing of such tools should be done as fol- 

 lows : The entire surface of the glass is painted witli rouge and water, and 

 tile machine is set to give a "normal" stroke, i. e., one by which the tool is made 

 to cover the entire surface of the mirror as uniformly as possible (without an 

 excess of action on any zone) as the glass revolves; the machine is run extremely 

 slowly, and the setting of the transverse slide is changed often ; after [tressing the 

 tool foi- an houi' or two in this way, polishing or figuring is to be begun. 



CHAPTER XI. 

 TESTING AND FIGURING SPHERICAL MIRRORS. 



Before describing the work of figuring concave mirrors, which is done with 

 polishing tools, it will be necessary to consider methods of testing. The principles 

 involved in testing concave minors at their center of curvature by Foncault's 

 method have been thoroughly ex[)lained and illustrated bj' Draper on pages 13-19 

 of his book, and by Dr. Common in his book On the Construction of a Five-Foot 

 Equatorial Reflecting Telescope. Foucault's original [)aper on this subject may be 

 found in Vol. V of the Annals of the Paris Observatory. 



All mirrors, when being tested, ai-e placed on edge, so that the axis of figure 

 is neaily horizontal, large mirroi's being suspended in a wide, flexible steel band, 

 lined with soft paper or Brussels carpet; for glass mirrors lai'ger than 30 inches in 

 diameter it is very desirable to have the grinding and polishing machine so con- 

 sti'ucted that the glass can be tniiied down on edge for testing, in the manner shown 

 on Plate ir, without lemoving it from the machine. A 30-inch glass mirror 4 

 inches thick weighs about 260 pounds; miirors larger than this are diflicult 

 to handle without suitable mechanism. 



A small, brilliant source of light, or " artificial star" may be })roduced by pla- 

 cing in front of the flame of an oil lamp a thin metal i)late in which a very small 

 [)inlio]e has been bored. If the ilhuninated pinhole l)e placed about an inch to one 

 side of the principal axis of the mii'ror, and at a distance from the mirror equal to 

 its radius of curvature, a reflected image of the pinhole will be formed on the 

 other side of tlie axis, and at the same distance from it and from the mirror as 

 the corresponding distances of the piidiole itself. If the surface of the mirror 

 is perfectly s})herical, and if theie are no atmospheric disturbances in the course of 

 the rays, the reflected image, when examined with an eyepiece, will be found to be 

 a pei'fect reproduction of the pinhole, with the addition of one or more diffiaction 

 rings around it, minute details of the edge of the pinhole apftearing as exquisitely 

 sharp and distinct as when tlie pinhole itself is examined with an eyepiece. If the 



