(COPYRIGHT.] [All Richts Reserved.] Photographing the Heavens. (By Sir Robert S. Bali, Astronomer Royal For Ireland.) [ALL RIGHTS RESERVED.]

Te Aroha News, Volume VII, Issue 430, 21 December 1889, Page 6

(COPYRIGHT.] [All Richts Reserved.] Photographing the Heavens. (By Sir Robert S. Bali, Astronomer Royal For Ireland.) [ALL RIGHTS RESERVED.]

1 bare say that a number of my readers will be surprised to learn that the beaven s contain multitudes of invisible stars. There are, of course, a few thousand stars that can be seen by the unaided eye, there are many millions of stars that can be seen with the aid ot a great telescope ; but over and above all these there are a myriad of stars that not even the keenest of eyes applied to the biggest and most powerful of telescopes will show a trace of. Does not this seem rather a puzzle ? If we cannot see the stars under any circumstances, how do we know that they are there ? Most assuredly we cannot feel their warmth, and as the 3tara arc always silent they can make no impression on our sense of hearing, while our two remaining senses of taste and of smell are obviously not of the slightest assistance in the study of the heavens. Yet, notwithstanding that we fail to perceive these stars directly, we have the most undoubted evidence of their existence. A star may be so faint that the beam of light it emits is too feeble to render it visible even with all the aid that great tele* scopes can give. The instrument collects together the rays and concentrates them so that they are all brought to a point on the sensitive*retina at the back of the eye. If the rays happen to be sufficiently powerful to act on the very cTelicate nerves of the eye, a message is sent trom the eye to the brain and the star is seen. If, however, the rays are too weak to excite the retina, then the star is of course not perceived. Under these circumstances, the only hope of discovering the star is to substitute for the retina something which shall be more sensitive to light. It has been found that the rays from one of these very faint stars when concentrated on a point of a good photographic plate will suv.., edin making an impression, provided that sufficient exposure has been given. The image on this plate can be developed and then the star is revealed. It can therefore be readily imagined that the U3e of photography is now essential to astronomical progress. It affords us the means of depicting tin magnitudes and the places of stars which must otherwise have been quite unknown to us. Some one may feel desirous to ask the same question as wa3 put to me by an intelligent lad when I was describing to him the way we proceeded : * How can you take photographs of the stars ?' he said, ' for you can only see them at night when it is quite dark, and yet everybody knows that photographs can only be taken when there is plenty of light.' To explain this difficulty you muat understand what goes on inside the camera when the operation is in progress. The glass lenses produce on the plate an image or picture of the object to be represented. In bright daylight it is the reflected sunlight which illuminates the object and enables it to form the picture. If the object has light of its own, a3 a star has, then thi3 light will form the necessary image on the plate and a photograph may be obtained. It is, however, essential that all light from every source except that coming directly from the object itself shall be carefully excluded, for this would act on the plate and obliterate the image. Indeed, when taking photographs of the stars we have to take special precautions to exclude stray light. I have heard of a plate having been spoiled by the fact that while it was being removed from the telescope after the stars had recorded their impression a carriage drove up the avenue and a flash from the carriage lamp chanced to fall on the plate and completely destroyed the faint images of the stars. Of course when once the plate has been developed it is then as permanent a3 a photograph taken in the ordinary way. Every photographer knows the great importance of giving to hia picture the proper length of exposure. He can now j use plates of p uch sensibility that a single second, or even much less than a second, will suffice for the most exquisite portrait or landscape. The brighter the light the shorter the exposure. On the other hand, ,\vhen the available light i 3 very weak a proportionately longer time is necessary. The light from some of the stars is so faint that the exposure is continued for a period which would astonish most photographers. Even though the plates be the most sensitive that can be manufactured, it is often advantageous to expose for a period, not of seconds or of minutes, but actually of hours. On some of the wonderful pictures which Mr Isaac Roberts has obtained, stars will be seen that have taken not le3S than four houra to engrave their little marks on the plate. Endeed, with every increased minute of exposure more and more stars, apparently without end, succeed in leaving their impressions. I have now before me a photograph taken by the astronomer just mentioned. It represents a small part of the constella- ' tion of the Swan. The picture is about as large as the page of a copybook, and it is bo crowded with stars that ib would puzzle you to count them ; but they have been counted by a patient person and the number is found to be about sixteen thousand. Many of these stars are too faint fco be ever seen-, in, the greatest of telescopes. . An attempt is now being made to obtain a number of photographs which shall cover the whole extent of the heavens. The ta3k •» indeed an immense oue. If the plates to be used were the same size as those of which I have just been speaking it would require about ten thousand of them to represent the entire sky. The great survey has therefore -to be divided among a number of observatories, some of which are to be in the Southern Hemisphere, for we must include pictures of the Southern Cross and the other constellations of the South^ which we can never see from this part of the world. >Some splendid telescopes have been specially constructed for the. purpose, and in a few years we may expect to see the survey in a fair

way towards completion. I may mention that the great difficulty encountered in taking the photographs is that of keeping the star always at the same point of the plate for the hour or more that the exposure is to last. The telescope is moved smoothly under electrical guidance from a clock so as to follow the star ; the attendant astronomer must at the same time patiently watch the movement with the aid of another telescopo,and incessantly guard against the slight irregularities from which no purely mechanical contrivance can be entirely free. When the great survey has been finished I we shall be in possession of a chart of the whole sky, embracing millions of stars, each accurately depicted in its exact place. It will be natural to ask what gain to knowledge may be expected from so elaborate an undertaking. Why should we go to great trouble and to great expense for the purpose of charting millions of stars, many of which we can see nothing more than that they are mere points of light ? I will mention one interesting branch of astronomy on which such a work as this great photographic survey seems destined to throw light. It is a magnificent fact that our sun, , bearing: with him his whole train of planets, of which our earth is one, is sweeping on through space with a majestic motion. There is only one way in which we can learn the particulars of this motion or discover the direction in which the sun is journeying or the speed with which his motion is performed. Our only knowledge on these points must be obtainod from looking at the stais. A mariner, when drawing near porb at night, is guided by the two entrance lights of the harbour. As he approaches he see 3 these two lights separate, while when he leaves the harbour at night the further he gets away the closer do tho two lights apj pear to draw together. In a similar manner the astronomer observes the lights of the universe to learn whither tho great solar system is tending. His beacons are the stars, and in that region towards which the sun is approaching the stars seem to spread away from each other, while in that opposite region from which the &un ia retreating the stars seem to draw in more closely together. Indeed, it is only by observing these changes in the apparent places of the stars that we learn the fact of the sun's motion. To investigate the subject properly we must employ a large number of stars, and we must wait a very long time in order to measure their displacements with precision. This is exactly the kind of work in which the photograph may be expected to help us. Every plate will bo carefully treasured up, to be compared with the plates taken in subsequent years, and from the changes in the apparent places of the stars we shall learn with more accuracy than we '.have hitherto been able to acquire the true facts with regard to that mighty voyage through 3pace on which our sun and all his system is journeying. But there are many other aids which photography renders to us in our efforts to learn more of the skies. One of tho most difficult problems is that of determining the distance of stars. This depends upon very careful measurements made between a star and other stars in its neighbourhood. It often takes an hour to measuie the apparent distance between two stars in tho ordinary way, but in that hour the telescope might much more profitably be employed in taking a photograph on which a hundred stars or perhaps more would be found, and tho distance between every pair of which would admit of being determined with accuracy. Still other and more marvellous applications of photography to the heavens have been made. The greatest telescopes are often directed to the observation of the nebuhu or stains and patches of light situated at incredible depths in space. There are thousands of these objects known to those astronomers who assiduously cultivate this branch of their science, and the nebuhu are all so faint that, with a single exception, they arc invisible to the unaided eye. The exception is the Great Nebula in Andromeda, which appears like a dull spot of light, just visible as a glimmer on the darkest night. For ages this object has attracted the attention of astronomers, and numerous drawings have been made of it. But the nature of the Great Nebula was never understood until a few months ago, when Mr Isaac Koberts obtained a marvellous photographic picture of the nebula after an exposure of four hour?. It was then seen to be composed ot a number of rings of gaseous material, apparently in a highly heated state and doubtless going through some sreat process of transformation. Hardly less wonderful is the photographic picture of Pleiades. This well-known group of little stars exhibits no nebulosity in the telescope, bub after a long exposure a nebula is depicted on the plate. Here photography has been the means of telling u» of the existence of a mighty nebula surrounding the Pleiades, but which was quite unknown till tho photograph showed it. Even now it has never been actually seen in the telescope. But I must conclude this \eiy brief sketch, and merely add that there are enormous developments of photographic work in the heavens which I have not been able to mention.