22 THE MODERN KEFLECTING TELESCOPE. 



eyepiece be moved outside aud inside of the focus, the expanded disk iu both cases 

 appeals perfectly rouud. Nothing cau be more impi'essive than to see such a 

 leflected iraafi'e produced by a fine spherical mirror having a radius of curva- 

 ture of 100 feet or more. Several such mirrors of 2 feet aperture have recently 

 been finished here. 



The use of an eyepiece is interesting for such experiments as that just 

 described, and is impoi'tant as a check upon the test with an opaque screen. The 

 latter test, however, which I shall call the knife-edge test, is used almost exclusively 

 for mirrors of all forms ; it is far more serviceable than the eyepiece test in deter- 

 mining the nature and position of zonal ii-i-egularities, and is far moi'e accurate in 

 determining the radius of curvature either of a mirror as a whole, or of any zones 

 of its surface. 



If the eye be placed just behind the reflected image of the illuminated pinhole, 

 so that the entire reflected cone of light enteis the pupil, the polished, unsilvered 

 mirror surface is seen as a brilliant disk of light. Let an opaque screen or knife- 

 edo-e be placed in the same plane through the axis as the pinhole, and be moved 

 across the reflected com from the left, and just in front of the eye ; if a dai'k shadow 

 is seen to advance across the raiii'or from the left, the pinhole and knife-edge are 

 inside of the best focus, and must be moved together away from the mirror; 

 if, however, with the knife-edge still moved across from the left, the shadow 

 advances across the mirror from the right, pinhole and knife-edge are outside of the 

 focus and must be moved toward the mirror. By repeated ti'ials a position is found 

 from which the shadow does not appeal- to advance from either side, but the miiTor 

 surface darkens more or less uniformly all over : this is the position oi' plane of the 

 best focus, and it is with this position of the knife-edge that irregulaiities of the 

 surface, if any exist, are seen in most highly exaggerated relief ; with this position of 

 the knife-edge, the mirror, if perfectly spherical, is seen to darken with absolute uni- 

 formity all over as the sci-een is moved across the focus, and the impression of a per- 

 fectly plane surface is given to the eye. 



If, however, the mirror is not perfectly spherical, but contains several zones of 

 slightly different radii of curvature, a very common case, these zones will appear as 

 protuberant or depressed rings on an othei-wise plane surface. The reason for this 

 is evident; the light from some parts of such zones is cut off by the knife-edge 

 before, from other pai-ts after, the illumination from the general surface is cut off; 

 the surface is therefore seen in light and shade, i. e., in enormously exaggerated 

 relief. The mirror must be regarded as being illuminated by light shining very 

 obli(|uelj^ along the surface from the side opposite that from which the knife-edge 

 advances across the focus. The interpretation of lights and shades becomes easy 

 after a little experience; not only is the character of a zone — whether it is an ele- 

 vation or depression — readily seen, but its diameter and its width are readily 

 determined. 



If the disk of glass is of sufficient thickness and of proper quality, and if 

 attention has been given to the uniform rotation of the turntable and to the pro- 

 tection of the glass from abnormal conditions of temperature dui-ing grinding and 



