A SILVERED GL A S S TE LE S C PE. 17 



After having gained from the preceding paragraphs a general idea of the vakie 

 and nature of these tests at the centre of curvature, a more particular description 

 of their use is desirable. M. Foucault in his methods first brings the mirror to a 

 spherical surface, and then by moving the luminous pin-hole toward the mirror, 

 and correspondingly retracting the eye-piece or opaque screen, .carries it, avoid- 

 ing aberration continually by polishing, through a series of ellipsoidal curvatures, 

 advancing step by step toward the paraboloid of revolution. The length of the 

 apartment, however, soon puts a termination to this gradual system of correction, 

 and he is forced to perform the last steps of the conversion by an empirical process, 

 and eventually to resort to trial in the telescope. 



With my mirrors of 150 inches focal length, demanding from the outset a room 

 more than 25 feet long, this successive system had to be abandoned. It was not 

 found feasible to place the lamp in the distant focus of the ellipse — the workshop 

 being less than 30 feet long — and putting the luminous source on stands outside, 

 introduced several injurious complications, not the least of which was currents in 

 the layers of variously refracting air in the apartment. In a still room the density 

 and hygrometric variations in its various parts only give rise to slight embarrass- 

 ment. The moment, however, that currents are produced, satisfactory examination 

 of a mirror becom(;s difficult. The air is seen only too easily to move in great 

 spiral convolutions between the mirror and the eye, areolae of aberration appear 

 around a previously excellent image, and were it not for the second test, any de- 

 termination of surface would be impossible. By that test the real deviations from 

 truth of figure can be distinguished from the atmo- 

 spheric, and to a practised eye sufficient indications ^^^^^^^^^^^^^^ 

 of necessary changes given. Such a movement as ^^^PH^^^^^^^^B 

 that caused by placing the hand in or under the line ^B^- 3^^^H 

 of the converging rays, will completely destroy the ' ^^^^^' -'^3^W| 

 beauty of an image, and by the second test give ^^^^m ^ Sfflfj 

 origin in the first case to the appearance Fig. 19. ' ^Sgjj/B^ff-: -^^ 

 In order to be completely exempt at all times from wK^^^^^'' - ''^^B 

 aerial difficulties, it is desirable to have control of a ""^^Bfer - -^tiHi^H 

 long underground apartment, the openings of which ^Hkh^^^^^fll^^l 

 can be tightly closed. As no artificial warmth is atmospheric Motions, 

 needed, there is the minimum of movement in the 



inclosed air, and conclusions respecting a surface may be arrived at in a very short 

 time. The mirror may also be supported from the ground, so that tremulous vibra- 

 tions which weary the eye, and interfere with the accuracy of criticism, may be 

 avoided. 



Driven then from observing an image kept continually free from aberration, 

 through advancing ellipsoidal changes, it became necessary to study the gradual 

 increase of deformation, produced by the greater and greater departures from a 

 spherical surface, as the parabola Avas approached. It Avas found that a sufficient 

 guide is still provided in these tests, by modifying them properly. 



The longitudinal aberration of a mirror of small angular opening is easily calcu- 

 lated—being equal to the square of half the aperture, divided by eight times the 



3 June, 1864. 



