ASTRONOMICAL PHOTOGRAPHY.

ASTRONOMERS have from the first introduction of photography used this art, which to them of all others promised to be the most useful. The many difficulties in the way of obtaining accurate observations of heavenly bodies, and the limited time during which such observations could be made and recorded in a sufficiently accurate manner, especially in such cases as solar eclipses, gave great importance to the plan of using this limited time to obtain a permanent picture that could afterwards be observed, scrutinised, and dealt with under the most favourable conditions, as if it were the image of the real object seen in the telescope.

The immense advantage that such a plan offered prompted astronomers to develope and apply photography as fully as possible in this direction.

Within one year of the publication of the details of the daguerreotype process, Draper, in America, had taken a picture of the moon. But little beyond merely experimental work was done in this way.

With the introduction of the collodion process in 1851 an enormous advance was made, and from this time astronomical photography became a firmly established art. Many workers soon brought out all the capabilities of this process, and on such objects as the sun, the moon, and some of the brighter stars, much good work was done; some of the results obtained by the earlier workers remaining unrivalled, but the limit to further advance was reached, and persistent effort failed to carry the capabilities of the process further. And so matters remained until the introduction of the gelatine dry plate.

With the introduction of this new process all the technical difficulties that surrounded the old process disappeared, and the astronomer found himself in possession of a method of observation and record, not indeed new in principle, but so enormously superior to that previously used, that had not this method been the result of a process of evolution, but suddenly put in his hands in its present perfected state, he would certainly have deemed it as great a boon as that given by the invention of the telescope itself.

The object of this paper is to show how this new method is superior to the old, what has been already done, how in many cases

by it the old method of eye observation may be with advantage entirely superseded, and above all how important it is that the work that is now possible should be at once put in hand.

In using the collodion wet plate process the manipulative skill required was considerable. The process was a continuous one. The preparation of the plate, the exposure, and the development had all to follow in proper time. This involved a state of preparedness as regards apparatus and chemicals that required unremitting care on the part of the astronomer, who could not choose his own time for exposing the plate, but had to wait often night after night till the opportunity came. The marvel is that such excellent results were obtained by the workers with this process. It speaks much for the care and skill that was given. In addition to these cares, the exposure of the plate had always to be limited to the time it would remain moist. This defect, together with the great comparative want of sensitiveness, limited the application entirely to objects bright enough to produce an image in this time. The gelatine dry plate process differs in many essential points. The plates can be prepared beforehand under the best conditions; the exposure may be made at any time, and for any length of time, continued even, if necessary, night after night; and the development may be done at any time after exposure. Those advantages alone would place this process far above the other for convenience, but they are as nothing in comparison with the greater amount of sensitiveness that this process gives, amounting to, at the least, thirty or forty times as much. It is in this respect that the dry plate is so superior to the wet. Manipulative difficulties are as nothing; they can be, and were in the case of the wet plate, overcome by care; but the want of sensitiveness was the fatal defect of the wet plate. For everything, when sufficient light was available, it was as good as any other-in some respects possibly better; but the telescope could always show more than it could photograph.

With the gelatine plate the greater sensitiveness, combined with such long exposures as may be given, enables photographs to be taken not only of all the objects that the telescope is capable of showing to the eye, but of objects that are too faint to be seen, and thus to render these otherwise invisible objects visible.

This very remarkable result prompts some interesting inquiries into the difference between observations made directly, and those made indirectly by means of photography. In making his apparatus the photographer has, consciously or not, produced such an arrangement as amounts almost to a direct copy of nature's construction of the eye. I do not know if the comparison has ever been made before, but it is extremely interesting to note how very closely art has imitated nature to produce similar ends.

Both the apparatus of the photographer and the eye have optical

means of producing an image. Both have means of altering the amount of light going through this lens or optical arrangement by the stop in the lens and by the iris in the eye. Both have means of altering the focus of the image by shifting the lens in one case, and by altering the curves of the lens in the other. Both have a dark chamber to keep out other light than that needed to form the image, with means to cover the lens if needed; and both have a sensitive screen on which the image falls, and where it is dealt with.

It is at this point where the similarity ceases, though the comparison between the sensitive film and the retina might be carried further.

In the eye the retina evidently acts as a transmitter of the sensations produced by the image to the brain, where such sensations are recorded, so that the power of the eye is limited by the power of the brain to record, as well as by the power of the retina to perceive. In comparison with the sensitive film, the eye as an observing and recording instrument has two serious defects: (a) If the amount of light from an object is insufficient to excite sensation, this object does not become visible, no matter how long it may be gazed at. (b) The eye can only deal with the image piece by piece. The seeing power of the eye, as regards the angular extent of the field taken in, is very great; but the observing power of the eye is limited to a very small central portion, so that though a great field may be seen in a general way, it can be observed only by a long process of piece by piece investigation. This does not permit different parts of an image to be compared at the same instant.

In those respects where the eye breaks down the sensitive film excels. In dealing with faint objects the lack of light can be made · up by sufficient exposure, so that the effective light is not, as in the eye, that forming the image, but the total accumulated light falling during the time the exposure is prolonged. It is in this way that with a certain sized telescope all the stars in a field of given size, visible to the eye, can be photographed with a certain exposure of the sensitive plate; then, by increasing the length of exposure beyond this point, if stars are in the field of view too faint to be seen, they will yet be photographed and thus rendered visible. And this will hold good no matter how large the telescope that may be used. With the most powerful telescope in existence, supposing, as we may safely do, that the number of stars of decreasing degrees of brightness is infinite, then therewill always be stars that can be photographed by this telescope that would otherwise for ever remain invisible, and so of nebulæ.

It is certain that such a means of examining the heavens would have a great bearing on questions concerning the distribution of the nebulæ and stars, and on the architecture of the heavens. It must add enormously to our knowledge in this direction.

VOL. XXI.-No. 120.

R

In dealing with a large image the sensitive film has no preference. The action takes place over the whole extent of the sensitive film, no matter how complex this image may be, and all parts are recorded with fidelity at the same time.

This power of photography in this respect is wonderfully shown in the photographs of the sun's surface by Janssen, where delicate differences in the appearance of the surface that would be quite beyond the eye are shown with perfect fidelity as they exist at the particular small fraction of a second during which the exposure lasts.

And it is also shown in the photographs of the Orion nebula taken in 1883, where all the most delicate structure of this most wonderful object is shown in proper relative intensity throughout, in a way that it would be simply futile to attempt to imitate by eye and hand.

Through the fact that rays that are most energetic on the sensitive film are not those that are most energetic in exciting the sensation of light in the eye, there will be a certain difference between the appearance of stars and other objects in which red or blue light predominates. A great deal has been made of this possible difference, but in practice it is very much less than has been expected, and the effect is only to make a slight difference between what is seen and what is photographed, the comparison between one photograph and another not being affected. To meet the case it will be quite sufficient, and as convenient, to compare stars or nebula by the photographic magnitude, as measured by their effect on the sensitive film, as it is to use magnitudes got by eye estimation, even when assisted by any of the photometers now in use.

The instruments used by the astronomer in photographing the various heavenly bodies differ in power according to the purpose for which they are intended to be used.

The immense difference between the brightest and the faintest object that has to be photographed is hardly to be realised. In the case of the sun the light is so great that special means have to be used to reduce the action to a very small fraction of a second of time; while with such a nebula as is just to be seen in a moderate-sized telescope, the light may be so faint as not to exceed the one twentythousandth of that received from a candle (of the usual standard size) shining at a distance of one quarter of a mile. This is according to an estimate made by Dr. Huggins in his spectroscopic work on the constitution of the nebulæ. Such different objects require different instruments, as can well be imagined.

Speaking generally, the image that the telescope produces, and which is in eye observations examined by aid of an eyepiece, is that generally photographed by placing the sensitive plate in the exact point where this image is formed.

In the case of reflecting telescopes, where the rays of light are

not dispersed by the reflection, this image is as perfect for photography as for ordinary observation.

With refracting telescopes this does not hold good; the image made for visual purposes is not that best suited for photography, and if such a telescope is used for this purpose, the best focus for the photographic rays is found by experiment. The image in this case can never be so good as in the reflector.

It is, however, possible to make the refractor as well fitted for photographic work as for visual, but in this case such an instrument is of little or no use for eye observation. In the case of large telescopes, primarily intended for ordinary work, the correction required to the lens to adapt it to the purpose of photography can be made by another lens placed in front of the object glass. Such is the plan that is to be adopted with the great Tick telescope, now being erected at Mount Hamilton, California. Object glasses specially made for photography are very excellent in work. The magnificent photograph of the moon produced by Rutherford in 1864, as well as the wonderful star charts of the Pleiades he also made, were made with such a lens (of about nine inches aperture). Dr. Gould used a similar lens in his photographic work at Cordova; and the Brothers Henry of Paris made in 1885 such a lens of about thirteen inches diameter. With this lens they have taken the beautiful photographs that all the world has heard of.

Both the refractor and the reflector are available for photography: which of the two telescopes will give the best results is yet an open question. For ordinary or eye observation the relative merits are so evenly balanced that outside considerations often determine the matter either way. The very much greater cost of the refractor will always be an important consideration in favour of the reflector, while the capacity for practically unlimited increase in size and power makes the latter the only telescope for a certain class of work.

In moderate sizes the photographic work done by the refractor is very fine from the absence of air currents in the tube, and from the optical perfection of the image. Reflectors of similar size have not yet produced such fine star or planetary photographs, chiefly because the image in the reflector is deformed by the arms that are used in the ordinary construction to carry the small mirror or the photographic plate, as the case may be. This is a defect that can be entirely removed in two ways, either by using a large plane mirror in front of the concave mirror, perforated so that the image may be formed at the back of the plane, or by placing at the mouth of the tube of the telescope a parallel plate of glass to carry the small mirror or the plate. This plate of glass can be made a lens of extremely flat curves as being more easily made, and the slight effect on the light passing through can be utilised either to render a spherical mirror effective or in combination with a mirror figured to match effect