A S I T; \^ E R E D G i, A S S T E L E S C > i I' E. 



17 



After liaviiii.- o-ained from the proc^Hliii..- paragraphs a general idea ,,f llic xaluc 

 and nature of these tests at the eei.trr ..f curvature, a more i)articuiar deseripli.m 

 nf their use is desirahh^. M. Fowault in his metlu)ds first hrin-s the mirror to a 

 spherical surface, and th.'ii by luovin- the Inminous i)iii-ii,,|,. toward the mirror, 

 and corresp(mdins-ly retracting tlu^ eye-piece or opa(|ue screen, carries it, avoid- 

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

 advancing step by step toward tlu- ])araholoi(i of revolution. The length of the 

 apartment, liowever, soon |)uts a termination to this graduid system of correction, 

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

 and e\(^ntually to resort to trial in tjie telescope. 



A\ith my mirrors of loO inches focal length, demanding from the outset a room 

 more than '>;■) feet long, this successive system had to hv abandoneil. It was not 

 found feasible to place the Lnnp in the distant focus of tlie ellipse— the worksjioj) 

 being less than ;3() 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 gi\c rise to slight embarrass- 

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

 of a mirror beconu's difficult. The air is seen only too easily to move in great 

 spiral convolutions between the nurror and tlu^ eye, areola; of aberration appear 

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

 termination of surfoce would be impossil)le. IJy 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 

 that caused by placing the liand in or under the line 

 (jf the converging rays, will completely destroy the 

 beauty of an image, and by the second test give 

 origin in the first case to the appearance Fig. 19. 

 In order to lie completely exempt at all times from 

 aerial difficulties, it is desirable to liaM' control of a 

 long luiderground a])artment, the openings of which 

 can be tightly closed. As no artificial warmth is 

 needed, there is the mininnnn of movement in the 

 inclosed air, and conclusions respecting a surfice nuiy 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, nray be 

 avoided. 



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

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

 increase of deformation, produccul by the greater and great(-r de])artures from a 

 spherical surface, as the parabola was apjn'oached. It was found that a snftieieni 

 guide is still provided in these tests, by nu)difying tbiMu ](roperly. 



Kie. in. 



.Atmiisplierii; Mutious. 



The longitudinal aberration of a mirror of small angular op^'uii 



IS easU\' c 



Iv calcu 



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



3 June, 1864. 



