_THE PLANETARY NEBULA H.IV 1, AQUARII_] [Illustration: _Plate 65_ _THE NEBULA H.IV 74, CEPHEI_] [Illustration: _Plate 66_ _THE NEBULA H.II 207, PEGASI_] [Illustration: _Plate 67_ _THE SPIRAL NEBULA H.I 53, PEGASI_] [Illustration: _Plate 68_ _THE SPIRAL NEBULA H.I 55, PEGASI_] [Illustration: _Plate 69_ _THE PLANETARY NEBULA H.IV 18, ANDROMEDAE_] [Illustration: _Plate 70_ _THE NEBULA H.II 240, PEGASI_] FOOTNOTES: [1] Reprinted from _The Astrophysical Journal_, =11=, 325, 1900. [2] For a more complete history of this part of the subject, see Dr. Holden's articles in _Pub. Ast. Soc. Pacific_, =7=, 197 _et seq._, 1895. [3] The difficulties here referred to, about which a good deal has been written, seem to have had their origin in the fact that it was impossible, at the time of the preliminary trials, to provide the observer with an assistant, while the Crossley reflector is practically unmanageable by a single person. [4] _Mon. Not. R. A. S._, =48=, 386. [5] Kindly lent by the Astronomical Society of the Pacific. [6] _Mem. R. A. S._, =46=, 173. [7] _Mon. Not. R. A. S._, =48=, 280, 1888. [8] _Mon. Not. R. A. S._, =49=, 297. The construction here described is not followed exactly in the Crossley apparatus. The guiding eyepiece slides freely when not held by a clamp. Pin-holes for preventing fogging are unnecessary when red light is used. [9] It so happens that the tension of the vertical thread is such that it begins to slacken when the temperature falls to within about 2° of the dew point. The thread thus forms an excellent hygrometer, which is constantly under the eye of the observer. When the thread becomes slack, it is time to cover the mirrors. [10] _Mon. Not. R. A. S._, =48=, 352. [11] The following list includes all papers of interest: "Photographic Observations of Comet I, 1898 (Brooks), made with the Crossley Reflector of the Lick Observatory." A. J. No. 451, =19=, 151; see also _Ap. J._, =8=, 287. "The Small Bright Nebula near _Merope_," _Pub. A. S. P._, =10=, 245. "On Some Photographs of the Great Nebula in _Orion_, taken by means of the Less Refrangible Rays in its Spectrum," _Ap. J._, =9=, 133. See also _Pub. A. S. P._, =11=, 70; _Ap. J._, =10=, 167; _A. N._, 3601. "Small Nebulæ discovered with the Crossley Reflector of the Lick Observatory," _Mon. Not. R. A. S._, =59=, 537. "The Ring Nebula in _Lyra_," _Ap. J._, =10=, 193. "The Annular Nebula H. IV. 13 in _Cygnus_," _Ap. J._, =10=, 266; see also _Pub. A. S. P._, =11=, 177. "On the Predominance of Spiral Forms among the Nebulæ," _A. N._, 3601. "The Distribution of Stars in the Cluster _Messier 13_ in _Hercules_" (by H. K. Palmer), _Ap. J._, =10=, 246. "The Photographic Efficiency of the Crossley Reflector," _Pub. A. S. P._, =11=, 199; _Observatory_, =22=, 437. "New Nebulæ discovered photographically with the Crossley Reflector of the Lick Observatory," _Mon. Not. R. A. S._, =60=, 128.