Part 5

Before beginning an exposure the focus is adjusted by means of a high-power positive eyepiece. An old negative, from which the film has been partially scraped, is placed in one of the plate-holders, and the film is brought into the common focus of the eyepiece and the great mirror. The appearance of the guiding star, which varies somewhat with the position of the guiding eyepiece on its slide, is then carefully noted, and is kept constant during the exposure by turning, when necessary, the focusing screw of the eye-end. For preliminary adjustments a ground-glass screen is often convenient. On it all the DM. stars, and even considerably fainter ones, as well as the nebulæ of Herschel's Class I, are easily visible without a lens.

Plates are backed, not more than a day or two before use, with Carbutt's "Columbian backing," which is an excellent preparation for this purpose. During the exposure the observer and assistant exchange places every half hour, thereby greatly relieving the tediousness of the work, though two exposures of four hours each, in one night, have proved to be too fatiguing for general practice. At the end of the first two hours it is necessary to close the slide and wind the clock.

The brightness of the guiding star is a matter of some importance. If the star is too bright, its glare is annoying; if it is too faint, the effort to see it strains the eye, and changes of focus are not easily recognized. A star of the ninth magnitude is about right. In most cases a suitable star can be found without difficulty.

In such an apparatus as that described above, the amount by which the plate may be allowed to depart from its zero position is subject to a limitation which has not, I think, been pointed out, although it is sufficiently obvious when one's attention has been called to it. It depends upon the fact that the plate necessarily moves as a whole, in a straight line which is tangent to a great circle of the sphere, while the stars move on small circles around the pole. The compensation for drift, when the plate is moved, is therefore exact at the equator only.

Let the guiding star have the declination [Greek: delta]{1}, and let a star on the upper edge of the plate (which, when the telescope is north of the zenith, and the eye-end is on the north side of the telescope, will be the southern edge) have the declination [Greek: delta]{2}. Then if the guiding star is allowed to drift from its zero position through the distance d, the other star will drift through the distance d (cos [Greek: delta]{2} / cos [Greek: delta]{1}). If the guiding star is followed by turning the right-ascension screw, the upper edge of the plate, as well as the guiding eyepiece, will be moved through the distance d. Hence there will be produced an elongation of the upper star, represented by

e = d ((cos [Greek: delta]{2} / cos [Greek: delta]{1}) - 1)

from which d = (e cos [Greek: delta]{1}) / (cos [Greek: delta]{2} - cos [Greek: delta_{1}]).

Now, in the Crossley reflector, the upper edge of the plate and the guiding eyepiece are just about 3-2/3 inches, or 1°, apart. If e is given, the above formula serves to determine the maximum range of the slide for different positions of the telescope.

It has been stated farther above that the smallest star disks, on a good photograph, are sometimes not more than 2´´ in diameter, or in a linear measure, about 1/20 mm. An elongation of this amount is therefore perceptible. There are many nebulæ in high northern declinations, and there are several particularly fine ones in about +70°. If, therefore, we take [Greek: delta]{2} = 70°, [Greek: delta]{1}, = 71°, e = 0.05, and substitute these values, we find d = 1.0 mm, which is the greatest permissible range of the plate in photographing these nebulæ. Before I realized the stringency of this requirement, by making the above simple computation, I spoiled several otherwise fine negatives by allowing the plate to get too far from the center, thus producing elongated star images.

There is a corresponding elongation in declination, the amount of which can be determined by an adaptation of the formula for reduction to the meridian, but it is practically insensible.

On account of the short focal length of the three-foot mirror, the photographic resolving power of the telescope is much below its optical resolving power. For this reason the photographic images are less sensitive to conditions affecting the seeing than the visual images. On the finest nights the delicate tracery of bright lines or caustic curves in the guiding star is as clear and distinct as in a printed pattern. When the seeing is only fair these delicate details are lost, and only the general form of the image, with its two principal caustics, is seen. A photograph taken on such a night is not, however, perceptibly inferior to one taken when the seeing is perfect. When, however, the image is so blurred that its general form is barely distinguishable, the photographic star disks are likewise blurred and enlarged, and on such nights photographic work is not attempted.

The foregoing account of the small changes which have been made in the Crossley telescope and its accessories may appear to be unnecessarily detailed, yet these small changes have greatly increased the practical efficiency of the instrument, and, therefore, small as they are, they are important. Particularly with an instrument of this character, the difference between poor and good results lies in the observance of just such small details as I have described.

At present the Crossley reflector is being used for photographing nebulæ, for which purpose it is very effective. Some nebulæ and clusters, like the great nebula in Andromeda and the Pleiades, are too large for its plate (3-1/4 × 4-1/4 in.), but the great majority of nebulæ are very much smaller, having a length of only a few minutes of arc, and a large-scale photograph is required to show them satisfactorily. It is particularly important to have the images of the involved stars as small as they can be made.

Many nebulæ of Herschel's I and II classes are so bright that fairly good photographs can be obtained with exposures of from one to two hours; but the results obtained with full-light action are so superior to these, that longer exposures of three and one half or four hours are always preferred. In some exceptional cases, exposures of only a few minutes are sufficient. The amount of detail shown, even in the case of very small nebulæ, is surprising. It is an interesting fact that these photographs confirm (in some cases for the first time) many of the visual observations made with the six-foot reflector of the Earl of Rosse.

Incidentally, in making these photographs, great numbers of new nebulæ have been discovered. The largest number that I have found on any one plate is thirty-one. Eight or ten is not an uncommon number, and few photographs have been obtained which do not reveal the existence of three or four. A catalogue of these new objects will be published in due time.

Some of the results obtained with the Crossley reflector, relating chiefly to particular objects of some special interest, have already been published. The photographs have also permitted some wider conclusions to be drawn, which are constantly receiving further confirmation as the work progresses. They may be briefly summarized as follows:

1. Many thousands of unrecorded nebulæ exist in the sky. A conservative estimate places the number within reach of the Crossley reflector at about 120,000. The number of nebulæ in our catalogues is but a small fraction of this.

2. These nebulæ exhibit all gradations of apparent size, from the great nebula in Andromeda down to an object which is hardly distinguishable from a faint star disk.

3. Most of these nebulæ have a spiral structure.

To these conclusions I may add another, of more restricted significance, though the evidence in favor of it is not yet complete. Among the objects which have been photographed with the Crossley telescope are most of the "double" nebulæ figured in Sir John Herschel's catalogue (Phil. Trans., 1833, Plate XV). The actual nebulæ, as photographed, have almost no resemblance to the figures. They are, in fact, spirals, sometimes of very beautiful and complex structure; and, in any one of the nebulæ, the secondary nucleus of Herschel's figure is either a part of the spiral approaching the main nucleus in brightness, or it can not be identified with any real part of the object. The significance of this somewhat destructive conclusion lies in the fact that these figures of Herschel have sometimes been regarded as furnishing analogies for the figures which Poincaré had deduced, from theoretical considerations, as being among the possible forms assumed by a rotating fluid mass; in other words, they have been regarded as illustrating an early stage in the development of double star systems. The actual conditions of motion in these particular nebulæ, as indicated by the photographs, are obviously very much more complicated than those considered in the theoretical discussion.

While I must leave to others an estimate of the importance of these conclusions, it seems to me that they have a very direct bearing on many, if not all, questions concerning the cosmogony. If, for example, the spiral is the form normally assumed by a contracting nebulous mass, the idea at once suggests itself that the solar system has been evolved from a spiral nebula, while the photographs show that the spiral nebula is not, as a rule, characterized by the simplicity attributed to the contracting mass in the nebular hypothesis. This is a question which has already been taken up by Professor Chamberlin and Mr. Moulton of the University of Chicago.

The Crossley reflector promises to be useful in a number of fields which are fairly well defined. It is clearly unsuitable for photographing the Moon and planets, and for star charting. On the other hand, it has proved to be of value for finding and photographically observing asteroids whose positions are already approximately known.

One of the most fruitful fields for this instrument is undoubtedly stellar spectroscopy. Little has been done in this field, as yet, with the Crossley reflector, but two spectrographs, with which systematic investigations will be made, have nearly been completed by the Observatory instrument-maker. One of these, constructed with the aid of a fund given by the late Miss C. W. Bruce, has a train of three 60° prisms and one 30° prism, and an aperture of two inches; the other, which has a single quartz prism, will, I have reason to expect, give measurable, though small, spectra of stars nearly at the limit of vision of the telescope.

The photogravure of the Trifid nebula, which accompanies this article, was made from a photograph taken with the Crossley reflector on July 6, 1899, with an exposure of three hours. It was not selected as a specimen of the work of the instrument, for the negative was made in the early stages of the experiments that I have described, and the star images are not good, but rather on account of the interest of the subject. At the time the photogravures were ordered no large scale photograph of the Trifid nebula had, so far as I am aware, ever been published. The remarkable branching structure of the nebula is fairly well shown in the photogravure, though less distinctly than in the transparency from which it was made. The enlargement, as compared with the original negative, is 2.9 diameters (1 mm = 13´´). The fainter parts of the nebula would be shown more satisfactorily by a longer exposure.

LIST OF NEBULÆ AND CLUSTERS PHOTOGRAPHED.

+----------------------------------------------------------------------+ |N.G.C.| [Greek: a]|[Greek: d]| Remarks. | | No. | 1900.0 | 1900.0 | | |----------------------------------------------------------------------| | | h m s | ° ´ | | | 185 | 0 33 25 | +47 47.3 |H II, 707 | | 205 | 0 34 56 | +41 8.2 |H V, 18 | | 221 | 0 37 15 | +40 19.0 |M 32 | | 224 | 0 37 17 | +40 43.4 |Great nebula in Andromeda | | 247 | 0 42 3 | -21 17.9 |H V, 20 | | 253 | 0 42 36 | -25 50.6 |H V, I | | 524 | 1 19 33 | + 9 1.0 |H I, 151 | | 598 | 1 28 12 | +30 8.6 |M 33 | | 628 | 1 31 19 | +15 16 |M 74 | | 650 | 1 36 0 | +51 4.0 |M 76 | | 891 | 2 16 15 | +41 53.6 |H V, 19 | |1023 | 2 34 8 | +38 38.0 |H I, 156 | |1068 | 2 37 34 | - 0 26.3 |M 77 | |1084 | 2 41 5 | - 8 0.0 |H I, 64 | | ... | 3 41 | +24 |Pleiades in Taurus | |1555 | 4 16 8 | +19 17 |T Tauri and Hind's variable nebula | |1931 | 5 24 48 | +34 10.1 |H I, 261 | |1952 | 5 28 30 | +21 57 |Crab nebula in Taurus | | ... | 5 30 | - 5 |Great nebula in Orion | |1977 | 5 30 27 | - 4 54.2 |H V, 30 | |2024 | 5 36 48 | - 1 54.3 |H V, 28 | |2068 | 5 41 37 | + 0 0.8 |M 78 | |2239 | 6 25 37 | + 5 1.1 |Cluster and nebula in Monoceros | |2264 | 6 35 | +10 0 |Nebula near 15 Monocerotis | |2287 | 6 42 43 | -20 38.4 |M 14 | | ... | 6 59 40 | -10 18.2 |New nebula in Monoceros | |2359 | 7 12 54 | -13 2.0 |H V, 21 | |2366 | 7 18 18 | +69 13.4 |H III, 748 | |2371-2| 7 19 6 | +29 41.0 | H II, 316-7 | |2403 | 7 27 9 | +65 48.9 |H V, 44 | |2437 | 7 35 24 | -14 35.3 |Cluster and nebula M 46 | |2632 | 8 34 | +20 |Præsepe cluster | |2683 | 8 46 29 | +33 47.8 |H I, 200 | |2841 | 9 15 6 | +51 24 |H I, 205 | |2903-0| 9 26 31 | +21 57 |H I, 56-57 | |3003 | 9 42 38 | +33 52.8 |H V, 26 | |3031 | 9 47 18 | +69 32 |M 81 | |3079 | 9 55 9 | +56 10.1 |H V, 47 | |3115 | 10 0 16 | - 7 14.0 |H I, 163 | |3169 | 10 9 4 | + 3 57.7 |H I, 4 | |3184 | 10 12 15 | +41 55.1 |H I, 168 | |3198 | 10 13 42 | +46 3.7 |H I, 199 | |3226-7| 10 17 59 | +20 24.1 |H II, 28-29 | |3242 | 10 19 29 | -18 5 |H IV, 27 | | ... | 10 21 7 | +68 58 |New nebula in Ursa Major (Coddington).| |3556 | 11 5 40 | +56 13.0 |H V, 46 | |3587 | 11 9 0 | +55 33.7 |Owl nebula, M 97 | |3623 | 11 13 43 | +13 38.4 |M 65 | |3627 | 11 15 1 | +13 32 |M 66 | |3726 | 11 27 56 | +47 35.8 |H II, 730 | |4244 | 12 12 29 | +38 22.0 |H V, 41 | |4254 | 12 13 45 | +14 59 |M 99 | |4258 | 12 14 2 | +47 51.6 |H V, 43 | |4303 | 12 16 18 | + 5 1.7 |M 61 | |4321 | 12 17 52 | +16 22.7 |M 100 | |4382 | 12 20 21 | +18 44.7 |M 85 | |4485-9| 12 25 40 | +42 15.3 |H I, 197-198 | |4501 | 12 26 56 | +14 58.5 |M 88 | |4536 | 12 29 20 | + 2 44.2 |H V, 2 | |4559 | 12 30 59 | +28 30.6 |H I, 92 | |4565 | 12 31 24 | +26 32.2 |H V, 24 | |4631 | 12 37 19 | +33 5.9 |H V, 42 | |4656-5| 12 39 6 | +32 42.8 |H I, 176-7 | |4725 | 12 45 33 | +26 3 |H I, 84 | |4736 | 12 46 13 | +41 39.5 |M 94 | |4826 | 12 51 49 | +22 13.9 |M 64 | |5055 | 13 11 20 | +42 33.6 |M 63 | |5194-5| 13 25 39 | +47 42.6 |M 51 | |5247 | 13 32 39 | -17 22.4 |H II, 297 | |5272 | 13 37 35 | +28 53 |M 3 | |5457-8| 13 59 39 | +54 50 |M 101 | |5857-9| 15 2 55 | +19 58.9 |H II, 751-2 | |5866 | 15 3 45 | +56 9.0 |H I, 215 | |5904 | 15 13 29 | + 2 27 |M 5 | |6205 | 16 38 6 | +36 39.0 |M 13 | |6218 | 16 42 2 | - 1 46.2 |M 12 | |6412 | 17 32 41 | +75 47.3 |H VI, 41 | |6514 | 17 55 43 | -23 2 |Trifid nebula in Sagittarius | |6523 | 17 57 43 | -24 23 |M 8 | |6543 | 17 58 35 | +66 38 |H IV, 37 | |6618 | 18 15 0 | -16 13 |M 17 Omega nebula | |6656 | 18 30 17 | -23 59.3 |M 22 | |6705 | 18 45 42 | - 6 23.3 |M 11 | |6720 | 18 49 53 | +32 54.0 |M 57 | |6853 | 19 55 17 | +22 27 |Dumb-Bell nebula | |6894 | 20 12 22 | +30 15.5 |H IV, 13 | |6946 | 20 32 48 | +59 48.0 |H IV, 76 | |6951 | 20 35 47 | +65 45.4 | | |6995 | 20 53 0 | +30 49.8 | | |7008 | 20 57 38 | +54 9.5 |H I, 192 | |7009 | 20 58 11 | -11 48 |H IV, 1 | |7023 | 21 0 30 | +67 46.2 |H IV, 74 | |7078 | 21 25 9 | +11 43.7 |M 15 | |7089 | 21 28 19 | - 1 16.0 |M 2 | |7099 | 21 34 42 | -23 38.0 |M 30 | |7217 | 22 3 24 | +30 52.3 |H II, 207 | |7331 | 22 32 30 | +33 53.9 |H I, 53 | |7448 | 22 55 7 | +15 26.6 |H II, 251 | |7479 | 22 59 56 | +11 47.0 |H I, 55 | |7537-4| 23 9 38 | + 3 59.4 |H II, 429-30 | |7662 | 23 21 5 | +41 59.2 |H IV, 18 | |7782 | 23 48 47 | + 7 24.8 |H III, 233 | |7814 | 23 58 8 | +15 34.5 |H II, 240 | |7817 | 23 58 52 | +20 11.6 |H II, 227 | +----------------------------------------------------------------------+

CATALOGUE OF NEW NEBULÆ DISCOVERED ON THE NEGATIVES.

+----------------------------------------------------------------------+ |No.|[Greek: a]|Precession|[Greek: d]|Precession| Description. | | | 1900.0 | | 1900.0 | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | 1| 0 0 27.4| +3.0732 |+20 34 57 | +20.048 |vS eeF | | 2| 0 32 7.7| 3.2795 |+47 55 29 | 19.855 |eF N | | 3| 0 32 8.1| 3.2801 |+48 1 22 | 19.855 |F vbM E140° | | 4| 0 32 9.3| 3.2776 |+47 37 24 | 19.855 |eF bM | | 5| 0 32 28.8| 3.2799 |+47 39 5 | 19.851 |B vE70° | | 6| 0 33 23.9| 3.2674 |+47 55 5 | 19.841 |eF vS | | 7| 0 35 43.1| 3.3009 |+47 46 18 | 19.810 |eF vS | | 8| 0 40 51.1| 2.9793 |-21 25 48 | 19.730 |18 vS R | | 9| 0 47 0.1| 2.9804 |-21 9 17 | 19.727 |16 vS bM 3 sep. parts | | 10| 0 41 16.2| 2.9781 |-21 29 43 | 19.723 |18 vS R bM | | 11| 0 41 16.7| 2.9792 |-21 15 2 | 19.723 |18 vS R | | 12| 0 41 29.7| 2.9798 |-21 3 8 | 19.719 |18 vS bM E50° | | 13| 0 42 4.4| 2.9633 |-26 0 7 | 19.711 |17 vS R bsw | | 14| 0 42 30.7| 2.9780 |-20 56 38 | 19.703 |18 vS bM E115° | | 15| 0 42 34.2| 2.9620 |-25 59 10 | 19.702 |17 vS N E160° | | 16| 0 42 37.6| 2.9776 |-20 58 28 | 19.701 |14 S E stell N | | 17| 0 42 39.7| 2.9772 |-21 1 54 | 19.701 |17 vS Spiral bM | | 18| 0 42 39.9| 2.9774 |-21 0 3 | 19.700 |18 vS Ring? | | 19| 0 42 40.5| 2.9770 |-21 3 55 | 19.700 |15 S Spiral N bM | | 20| 0 42 40.6| 2.9762 |-21 13 54 | 19.700 |18 vS R | | 21| 0 43 10.4| 2.9603 |-25 59 36 | 19.692 |18 vS R bM | | 22| 0 43 16.2| 2.9730 |-21 37 17 | 19.691 |18 vS dif | | 23| 0 43 27.1| 2.9613 |-25 40 21 | 19.688 |17 vS R N | | 24| 0 43 29.0| 2.9593 |-26 0 57 | 19.687 |18 vS R gbM | | 25| 0 44 10.8| 2.9714 |-21 30 29 | 19.676 |18 vS R | | 26| 0 44 26.6| 2.9735 |-20 58 35 | 19.672 |17 vS R bM | | 27| 1 18 30.9| 3.1475 |+ 9 27 25 | 18.887 |F S N | | 28| 1 18 53.5| 3.1475 |+ 9 24 28 | 18.875 |F vbM Spiral? | | 29| 1 19 11.3| 3.1474 |+ 9 21 53 | 18.867 |F vbM Spiral? | | 30| 1 19 30.7| 3.1467 |+ 9 14 18 | 18.857 |F bM E | | 31| 1 29 50.7| 3.2101 |+15 6 37 | 18.526 |pF E45° bp | | 32| 1 29 54.4| 3.2161 |+15 43 25 | 18.524 |F R | | 33| 1 30 20.9| 3.2127 |+15 17 38 | 18.509 |vF L R | | 34| 1 30 24.7| 3.2132 |+15 20 28 | 18.507 |pF S vF extension 135°| | 35| 1 30 35.9| 3.2153 |+15 32 2 | 18.501 |S pB pmb M | | 36| 1 30 54.7| 3.2176 |+15 43 1 | 18.491 |vvF vS | | 37| 1 31 5.0| 3.2179 |+15 43 38 | 18.485 |F S E95° | | 38| 1 31 15.9| 3.2159 |+15 30 44 | 18.478 |pF S R | | 39| 1 31 25.7| 3.2187 |+15 44 34 | 18.473 |vF S R | | 40| 1 31 44.8| 3.2194 |+15 46 49 | 18.462 |F L R gbM | | 41| 1 31 44.8| 3.2126 |+15 4 18 | 18.462 |F L gbM R | | 42| 1 32 5.9| 3.2158 |+15 20 54 | 18.450 |S pB E135° | | 43| 1 32 41.3| 3.2171 |+15 23 22 | 18.430 |vF S E45° | | 44| 1 32 48.8| 3.2156 |+15 12 27 | 18.424 |vF pL | | 45| 1 33 10.4| 3.2168 |+15 16 49 | 18.413 |vF pL gbM | | 46| 1 33 13.2| 3.2166 |+15 15 14 | 18.412 |p B R gbM | | 47| 2 14 10.2| 3.7341 |+41 50 8 | 16.715 |pF E135° | | 48| 2 14 26.6| 3.7349 |+41 49 1 | 16.701 |pB N R | | 49| 2 14 33.9| 3.7307 |+41 37 31 | 16.696 |B N | | 50| 2 14 36.7| 3.7313 |+41 38 24 | 16.694 |F | | 51| 2 14 55.0| 3.7506 |+42 24 20 | 16.677 |eF vS bM E135° | | 52| 2 15 6.2| 3.7517 |+42 25 6 | 16.668 |F gbM E130° Spiral? | | 53| 2 15 14.9| 3.7493 |+42 16 44 | 16.661 |F pmbM | | 54| 2 15 16.1| 3.7484 |+42 14 4 | 16.659 |F B{*}f | | 55| 2 15 38.4| 3.7666 |+42 55 0 | 16.641 |eF vS R | | 56| 2 15 43.8| 3.7503 |+42 13 58 | 16.637 |S F R | | 57| 2 15 56.5| 3.7724 |+43 5 24 | 16.626 |F E170° bsf | | 58| 2 16 1.0| 3.7539 |+42 20 55 | 16.623 |B S vbM E150° bnp | | 59| 2 16 6.4| 3.7403 |+41 44 51 | 16.619 |S F R | | 60| 2 16 9.7| 3.7408 |+41 45 26 | 16.616 |F S pmbM | | 61| 2 16 13.0| 3.7613 |+42 36 32 | 16.613 |pB vbM E150° Spiral? | | 62| 2 16 31.1| 3.7640 |+42 39 27 | 16.598 |eeF E50° | | 63| 2 16 34.5| 3.7412 |+41 42 6 | 16.595 |pB pmbM | | 64| 2 16 40.3| 3.7620 |+42 33 22 | 16.591 |B S pbM | | 65| 2 16 43.3| 3.7403 |+41 38 14 | 16.588 |pB E0° pmbM | | 66| 2 16 53.2| 3.7625 |+42 32 12 | 16.580 |vB S mbM | | 67| 2 16 57.8| 3.7567 |+42 16 48 | 16.576 |F triN npN | | 68| 2 17 13.8| +3.7403 |+42 22 37 | +16.563 |pB bs B{*}p | +----------------------------------------------------------------------+