APPENDIX I.

OBSERVATIONS BY PROF. W. C. BOND.

THE following pages contain such measures mostly of right ascension and declination directly made in 1847-48, of the stars in the nebula, as I have been able to find in the observing books. They have been reduced to arc, employing the value 9."800 of a revolution of the micrometer screw, omitting, however, the small correction for temperature, which does not appear to be very certain, and also the refraction correction, which cannot be strictly computed, as the time is not given. These corrections amount at most to a very few tenths of a second, and are not essential here.

After the observations for position, come some physical notes on the nebula, which have also been extracted from the observing books, for dates between 1847 and the commencement of Prof. G. P. Bond's systematic course of observations, the main subject of the present book.

OBSERVATIONS OF STARS IN THE NEBULA. 1847-48.

DIRECT MEASURES OF DIFFERENCES OF RIGHT-ASCENSION AND DECLINATION.

Besides the above, there are a few measures of angles of position and distance, made October 10th, 12th, 15th. These, however, are not given in original, in the observing books, but simply as results; they are passed over here, as they appear to refer entirely to brighter and well-known stars, and some errors exist in the copy, which alone I have seen. It is probable that they were not used in the preparation

of the Memoir.

The faint stars in the vicinity of the trapezium are not among them, and appear to have been given in the catalogue from estimates.

The remarks about the physical appearance of the nebula are in many cases interesting, and some extracts from them, including all where I have been able to make out with certainty the portion of the nebula observed, are now given.

1847, Dec. 6. The brighter parts are constantly sparkling with points of light, seen in favorable moments. No. 6, plain eye-piece, breaks the nebula into cumulus clouds.

1847, Dec. 24. [Some measures were made of points in the nebula; the following are definitely given as results.] From a to the edge of the preceding portion of the brightest part of the nebula, A AR. 10.03 [=98."2]. Breadth of the brighter portion of the nebula in the parallel of declination of a, 12.64 [=123."9]. Difference of declination of a and lower coast of the bay n. f. 8.29 [=81."2]; as a is almost exactly on the parallel of upper coast of this bay, this 8.729 is the breadth. [The coördinates of the terminus of the brighter part of the nebula apparently above are found to be a- -a。 = 0.0, 8—8。=−157."6; and those of the bottom of the bay south preceding, -77.3, -75."6.]

The fixed wire on a, in a parallel of declination, runs along the upper edge of the opening following the trapezium, past No. 5 [H. 110].

1848, Jan. 4. The preceding boundary of the horn is very sharply defined all the distance from below 24 (H. 123) to above 25 (H. 126); the turn towards 25 is firmly outlined and beautifully curved, equally above and below; the following side of the horn is by no means so well defined, but flies off in wisps of cirrus. The branch below is of the same character; this branch originates in the stronger light. There is undoubtedly considerable nebulosity enveloping the stars e, 4, 5, 6, and 7, but the sudden increased density of the "horn" is very marked and clearly defined.

The cam-shaped portion which is connected with the main in the s. p. direction from * 21 [H. 108], forms on the following side rather more than a right angle, terminating abruptly at 29 n. f., [H. 124?] the light is somewhat condensed to 21, but not strongly.

The brighter portions of the nebula are broken up into cumulus.

The northern nebula which we have brought in to-night is connected with the main by the preceding route, sweeping round with faint light. We do not resolve the nebula to-night; the stars are broad and blotty; ther mometer falling.

1848, Jan. 12. The brighter portion seems full of points of light at times. I have the utmost confidence in treating these minute points.

1848, Jan. 17. [Diagram by W. C. B.; marginal note by G. P. B.] "This diagram shows terminal angle of R. Huygheniana in AR. 150"."

1848, Feb. 7. From the star at the termination of the Messierian Branch the nebulosity radiates upward (south) and in the preceding direction, terminating in a cloudy appearance. The nebula passes on the following side of Orionis.

1848, March 22. It was uncommonly fine seeing just as twilight ended. I saw many stars in the neighborhood of the trapezium of Orion, and Sir John Herschel's star, the sixth star, stood out boldly. It is all but certain that this part of the nebula is composed of clusters of stars.

The first tendency of s. p. side from south cape is at a right angle to the s. f. side, provided we limit the region to the brightest portion; but the light is pretty strong in a southern direction, so that by taking the whole extent of the cape we lessen the angle to about 70 or 75 degrees.

There are other remarks with these, which are more or less obscure; and I am apprehensive that I have not, through my slight acquaintance with the nebula, got the sense of all which might be understood by a more competent editor. Some of Prof. W. C. Bond's notes upon the nebula are apparently not now to be found; at least I so explain the deficiency in places of the preceding observations of position, as compared with his catalogue. Yet in many cases, the catalogue positions may have been inserted by estimation, or perhaps have even been taken from the chart.

In connection with Prof. W. C. Bond's observations, I placed his original drawing (or what appears very certainly to be such) in the hands of Mr. Watts, to reëngrave. The difficulty of his task has been great, and the variations between his engraving and the former one considerable; but, as might be expected from so skilful and conscientious an artist (known to astronomers by the engravings in Vol. III. of these Annals), the present edition is much more accurate than the former. His principal care has been to render with fidelity the nebulous parts, any deviation in which would affect the appearance more sensibly than inaccuracies in the magnitudes of the stars, which are of less importance in the drawing.

Some very faint details in Prof. G. P. Bond's drawing, as, for instance, the very slight nebulosity about 2' preceding H. 136 = No. 848 of the present catalogue, are, with the somewhat greater optical power of the Chicago equatorial, more conspicuous; and I am led to the conclusion that Prof. W. C. Bond's drawing represents the nebula as seen in a very advantageous state of the atmosphere; while the later and more elaborate engraving of Prof. G. P. Bond certainly represents it as seen on usual good nights, with great precision.