THE MODERN REFLECTING TELESCOPE. 35 



uniformity all ovei-. As the paraboloidal surface nears completion, an elevated or 

 depressed center, a "turned up" or "turned down" edge, or protuberant or de- 

 pressed zones, can be seen and their character and exact position determined, with 

 precisely the same ease and cei'tainty with which similar iiregularities are seen when 

 a spherical miiTor is examined at its center of curvatioii with the knife-edge test. 



It should be noticed that even when the pinhole and reflected image are very 

 near each other, as they should be, yet both may be far out of the axis of the para- 

 boloid, if the mirrors are not properly adjusted or collimated; when this is the case 

 the miri'or surface, when seen with the knife-edge test, does not appear as a sui'face 

 of revolution, and cannot be properly tested. The mirrors may be collimated by 

 the following method, thus insuring that the pinhole and I'eflected image are both 

 exti-emely near the optical axis. 



The mirrors are set up approximately right by measurement. A ring about an 

 inch in diameter, with two fine threads stretched diametrically across it, one verti- 

 cal, one horizontal, is set up near the plane of the illuminated pinhole, the intersec- 

 tion of the threads marking the desired position of the optical axis. A light, 

 stiff ring is made, which fits closely over the edge of the paraboloidal mirror, at the 

 front; this ring can be slipped on and taken off as required. Two very fine bright 

 wires are stretched diametrically across this ring, one vertical, one horizontal ; these 

 wires should be as close as possible to the face of the mirror ; their intersection 

 marks the position of the center or vertex of the paraboloid. Two fine shoit lines, 

 one vertical, one horizontal, are scratched with a fine needle-point at the center of 

 the silvered face of the small diagonal plane miiroi-. The eye is now placed about 

 3 feet outside of the plane of the crossed threads, and an assistant changes the in- 

 clination of the small plane mirror, by means of three adjusting-screws at its back, 

 until the intersections of the threads, of the scratches, and of the wires are all 

 seen in exact coincidence. The assistant next changes the inclination of the para- 

 boloidal mirror (by means of three adjusting-screws at its back) until, with the 

 eye in the same position as before, the intersection of the threads, the intersection 

 of the wires, and the reflection of the intersection of the threads seen in the para- 

 boloidal mirror, all appear in exact coincidence ; the position of the axis of the 

 paraboloid has now been defined. No attention is paid to the large plane mirror 

 in this part of the work. The illuminated pinhole is now placed in position, and 

 the large plane mirror is adjusted (by means of thi-ee adjusting-screws at its back) 

 until the reflected image falls in the right position with reference to the axis and 

 pinhole. 



The frame which carries the paraboloidal mirror can easily be so designed that 

 this mirror can be removed and replaced repeatedly, while figuring it, without 

 sensibly disturVjing the adjustments. 



The difliculties of making short-focus paraboloidal mirrors of fine figure are 

 so greatly reduced when this method of testing is used that I believe that the gen- 

 eral adoption of this method by opticians would lead to such improvements in results 

 as to bring about a marked advance in the usefulness of reflecting telescopes. The 

 making of the large plane mirror which is necessary in this test becomes so simple 



