No portion of the heavens has been more carefully studied than that containing the Nebula of Orion. The monographs by Prof. G. P. Bond (Annals Harvard College Observatory, V.) and by Prof. E. S. Holden (Washington Astronomical Observations for 1878, Appendix I.) show the vast amount of material collected by eye observations. For a photographic study of the same region the following specimens are in the photographic collection of the Harvard College Observatory :

A. Artotype enlargement of the first photograph of the nebula taken by Dr. Henry Draper, September 30, 1880. Exposure, 51 minutes. B. Artotype enlargement of a photograph taken by Dr. Henry Draper, March 11, 1881. Exposure, 106 minutes.

C. The original negative taken by Dr. Henry Draper on March 14, 1882. Exposure, 136 minutes. This negative, except for a slight photographic blemish, is nearly identical with that from which D. was taken.

D. An enlarged glass positive of the second photograph taken by Dr. Henry Draper, March 14, 1882. Exposure, 137 minutes. This positive is a duplicate of that employed in making the paper prints, E. The two positives were taken, and any objects resembling stars, but not found on both, were assumed to be defects, and were painted out of the other positive by Dr. Draper.

E. Several artotype enlargements of the second photograph, taken March 14, 1882, by Dr. Henry Draper.

F. Carbon print of photograph taken by Mr. Common with his 3-foot reflector, January 30, 1883. Exposure, 39 minutes. Enlargement about 7 times.

G. Glass positive, a direct copy of the negative taken by Mr. Common with his 3-foot reflector, February 26, 1883. Exposure, 60 minutes.

Admirable material is thus furnished for a comparison of the results of photographic and eye observations of this region. The photographs of the stars which are common to F and to the catalogue of Professor Bond (Annals, V. 270) were first compared by a method closely resembling that adopted by Argelander for the study of variable stars. Table I. gives the stars which were selected for standards, with which the others are to be compared. Each star in the photograph was then compared with two of these,-one a little brighter, the other a little fainter. The differences were estimated in grades. The sum of the two differences gave a measure of the interval between the two comparison stars. It frequently happened that no difference in brightness was perceptible between the star to be measured and one of the comparison stars. The number of measures of star intervals between the comparison stars is therefore less than the number of stars compared. In Table I. the successive columns give for each comparison star a designation, and the number and magnitude in the Bond catalogue. The next column gives the photographic magnitude, found by a process which will be detailed below. This is followed by the number of comparisons between each star and that following it, and the mean value of this difference in grades. The last column gives the assumed brightness in grades, and equals the number of grades by which each star is fainter than the first on the list.

The light of each star measured was next reduced to grades by the assumed light in grades of the comparison stars. Two values were found, one derived from the brighter, the other from the fainter comparison star. In 50 cases the results agreed exactly, in 11 cases

they differed by one grade, and in one case only by two grades. The relation between the grades and the scale of magnitude of the Bond Catalogue was next found by grouping the stars by half-magnitudes. The middle points of each group are given in the first column of Table II., the number of stars in the group in the second column, and the mean of the corresponding values in grades in the third. Points were then constructed with the first and third columns as ordinates, and a smooth curve drawn through them. The comparative values of the

grades and magnitudes derived from this curve are given in Table III. Applying the results of this table to the last column of Table I. gives the fourth column of that table. The results for all the stars in the Bond Catalogue differing less than 1000" in right ascension and declination from ' Orionis, the brightest star in the nebula, are given

in Table IV. The first four columns give the number, difference in right ascension and declination from ' Orionis, and magnitude according to the Bond Catalogue. The fifth column gives the magnitude found as described above from F, the photograph of Mr. Common.