[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.

“The Spiral Nebula, H. I., 55 Pegasi,” Ap. J., 11, 1.

“Photographic Observations of Hind’s Variable Nebula in Taurus, made with the Crossley Reflector of the Lick Observatory,” Mon. Not. R. A. S., 60, 424.

“Use of the Crossley Reflector for Photographic Measurements of Position,” Pub. A. S. P., 12, 73.

“Discovery and Photographic Observations of a New Asteroid 1899 FD.,” A. N., 3635.

“Elements of Asteroid 1899 FD.” (by H. K. Palmer), A. N. 3635.

[12] Footnote added in 1908: This concluding paragraph, retained in the present publication for completeness, loses point in some particulars, because the photogravure referred to is not reproduced here. The heliogravure reproduction of the Trifid nebula is No. 55.

[13] Since then a photograph by Dr. Roberts has appeared in Knowledge, 23, 35, February, 1900.