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actinic focus may be stated in a few words. With the rack motion adjust the focus for distinct vision on the ground glass, and then mark the tube E, and also the sliding part of the telescope. Although very unlikely to be of the slightest use, unless taken with a reflecting telescope, a picture may now be taken ; it will at least serve to give some idea of the proper exposure. If the chemical and visual foci are not coincident, the image will have a blurred appearance. Before exposing the next plate, turn the adjusting screw so as to lengthen the tube about the sixteenth of an inch, and so proceed until, by the greater distinctness of the image, it is seen that the chemical focus is found. At every change of the focus a slight mark should be made on the tube, and when the true focus is satisfactorily determined the marks should be made distinctly visible; and in all future experiments with the same instrument the focus will be always at or very near the same place. Should it be found that the indistinctness increases, it will of course be necessary to try in the other direction.
The appearances arising from atmospheric disturbances are very much the same as when the object is out of focus; experience alone will enable the operator to determine from which cause the defect proceeds.
It is assumed that the telescope is provided with a driving clock; when such is the case every care should be taken that all the parts are clean, and when necessary oiled or greased, so that the motions may be as smooth as possible.
In photographs of the moon in the phases prior to and after the full, the side opposite to the sun is always too light, or burnt up,' while those parts near the terminator are often so dark that only the tops of the craters and peaks are visible, although in the telescope a clear and bright image can be seen. The cause of this must be that the exposure, if continued long enough to bring out all the eye can see on the darker side, would entirely obliterate the details on the brightly-illuminated portions of the moon's surface. Mr. De la Rue's suggestion as to why the dark side of the moon has so little actinic effect, has been already referred to. I would suggest that, as the light of the full moon is 100,000 times weaker than that of the sun, the twilight on the moon's surface must be very much less, and consequently the actinic effect of the light is lessened in the sa'e way as at a corresponding time on the earth.*
The question, then, of photographing the terminator is only one of time; and in order to remedy the defect spoken of to some extent, I have used diaphragms such as are shown in the drawing. In the tube E, openings are made on opposite sides, and wide enough to admit the diaphragms to be used without touching the tube. The diaphragm must be of the proper length and width to shut off the moon's light until the plate is ready for exposure. The shape of the diaphragm will suggest which form should be used, according to the moon's age. The exposure should be made with the full aperture for as many seconds as previous experiments have proved to be necessary for the bright side, and the diaphragm then gently moved and kept in motion, gradually approaching the darkened side. By this means the exposure may be regulated, and the great differences in the light and dark sides of the moon may be modified.
As to the processes employed, each experimenter must adopt the one he finds in his hands gives the best result. It seldom happens that two operators can produce the same effects with, apparently, the same chemicals. Experience has shown me that the ordinary patent plate glass (carefully selected, so as to be free from scratches and other defects) is preferable to the white patent plate, having found that after a time the surface becomes covered with a kind of dew or “sweat,” as it is termed, owing to the decomposition of some of the salts used in the manufacture. The collodion used was made for me by Messrs. Huggon & Co. of Leeds; it is very quick, free from structure, and suitable for iron development. I prefer to develope with an iron solution, using only sufficient to cover the plate; and with this developer and collodion, when the plate has been properly exposed, a negatire can be obtained which will not require intensifying afterwards. Not having had sufficient experience with pyrogallic acid, I cannot speak with confidence as to any advantage it may possess in giving fine texture to the negative. With the bath and collodion exactly in the proper state, there is no doubt that with this acid negatives may be made as quickly as with iron; but it is extremely difficult to have everything constantly in the best working order. Unless the greatest attention be given to this matter, the time of exposure is so much increased that iron, for this reason, must have the preference.
* In the absence of an atmosphere on the moon, there can of course be no twilight such as we have on the earth. I mean to say, that on those parts of the moon enlightened only by the oblique rays of the sun, the light is so diminished that the actinic effect is lessened as it is on the earth shortly before sunset and during twilight, when it is well known that a much longer time is required to obtain a photograph.
Upon the character of the image after development entirely depends the value of the enlargement to be made from it, and in this direction there is much room for improvement. Even in the best negatives yet made defects from this cause are very apparent. The microscopic photographs by Mr. Dancer have the finest texture, and will consequently bear greater magnifying than any other photographs I have ever seen, but the process by which they are made is not published.
The weather in this country is so very uncertain, and success in this branch of photography is so entirely dependent on the state of the atmosphere, that it is necessary to be always prepared to take advantage of a favourable night. I have a small cupboard placed in a convenient part of my house where there is a supply of water, and the temperature is always much above the air outside. This cupboard just large enough to hold a small glass bath fixed at the proper angle ready for use, also the bottles for collodion, bath, developing and fixing solutions, and other little requisites. This arrangement is so convenient that when there is a prospect of getting a negative I can set the telescope, prepare the plate, and take a negative in less than ten minutes. But when there is a chance of two or three hours' work an assistant is desirable, as the best results can only be obtained when one's attention is chiefly devoted to the careful adjustment of the apparatus connected with the telescope.
The convenience of the plan adopted may be judged of by the fact that on the evening of the partial eclipse of the moon, the 4th October last, in four hours I succeeded with the help of two assistants in taking no less than twenty negatives, and the telescope was several times disturbed to oblige friends who desired to see the progress of the eclipse through the instrument; but the apparatus was quickly readjusted, although, possibly in some cases with slight loss of definition in the negative, through haste. At a previous meeting, I described how these negatives were made, but it may be interesting to refer to the fact that while the fifteenth of the series was taken the telescope was at rest. The clock had been disconnected for readjustment, and it was forgotten when the plate was ready for exposure; consequently the moon had moved partly off the plate, and the negative shows a portion only; but the exposure was so short that the eye fails to detect any difference in the sharpness of this and the others, which were all taken when the clock had been watched and carefully regulated for the moon's motion. This fact is, I think, of some interest, as it shows that about the time of full moon, when the light is of the greatest intensity, pictures may be made with telescopes not equatorially mounted.
My telescope is a refractor of five inches aperture and six feet focal length, giving an image of the moon averaging about eleven-sixteenths of an inch in diameter.* The object-glass is of Munich manufacture, and is mounted by Mr. Dancer on the Sisson's or English plan with double polar axis. The hour circle is twenty-six inches in diameter, and is used also as the driving wheel, having teeth cut in the edge in which a screw works, and by which it is connected with the driving clock by a rod, and can be instantly disconnected by means of cam. The object-glass is an excellent one, and the mounting is everything that can be desired.
The negative taken direct in the telescope is but one step towards what we require—that is, the enlarged copy on paper. From the small negative a positive on glass must be made, say of twice or three times the diameter of the original. It will be quite unnecessary here to explain how the enlargement is to be made, but I may remark that the negative should be placed with the film side towards the copying lens, and the resulting positive copy must also be placed in the same way. The enlarged copy or negative will then give the true telescopic appearance of the moon. In the print of the full moon by Mr. Rutherford a mistake has been made, arising from the negative or positive copy having been placed the wrong way, and consequently the moon looks as if it had been photographed from the opposite side. The print is a very beautiful one in other particulars, but entirely worthless as a picture of the moon, as the eye can never see it as there represented.
I have sometimes taken two negatives on the same plate. It will be seen in the drawing that the dark slide is not quite central with the telescope, so that by reversing the plate after one exposure a second picture can be taken. In photographing the planets, Mr. De la Rue has allowed the object to move on for a few seconds, the telescope meanwhile being at rest, and thus four or five negatives can be taken in a very short time on the same plate. It has occurred to me that by having a frame made“ landscape way" instead of upright, and in place of having four clips such as K, there might be a kind of groove at top and bottom, so that after taking the first negative, and the light shut off, by moving the plate about an inch, at least three negatives might be taken on the same plate-or a “shifting back” might be adapted. The advantages of this plan are that different exposures might be tried, and the development continued for the one or two which promised the best results. This method would effect a great saving of time, which on a fine night is of much importance.
With the Barlow lens I have made some negatives which have shown that when the same care has been taken to find the actinic focus negatives of a much larger size may be made, and in a very short time. The image is increased from eleven-sixteenths to one-and-a-quarter-inch, and the time of exposure at full moon was two seconds. The fittings of the lens are so arranged that three different-sized negatives may be taken.
There are several other matters which it might have been desirable to refer to had time permitted, but they must stand over to a future occasion.
A. BROTHERS, F.R.A.S.
* The actinic focus is one-tenth of an inch longer than the visual.
ROYAL ASTRONOMICAL SOCIETY.
Fourth Meeting, February 9, 1866.
Annual General Meeting.
Secretaries-Rev. C. PRITCHARD and Mr. HODGSON.
Arthur Finch, Esq., and
Arthur Brewin, Esq., were balloted for and duly elected Fellows of the Society.
The President said :
“ Before proceeding with the business before us, I would for a few moments bespeak your attention. You will readily recall the great regret felt by us all on learning that the Medal awarded last year to Professor Bond, of the United States, did not reach that country until after his lamented decease.
“ You will, doubtless, therefore, be much gratified now to learn that Professor Bond, though he did not actually receive the Medal, was, some time before his death, made aware of the honour that had been conferred upon him, and even of the grounds on which the award had been made.
For being very desirous of not omitting the mention of any of his numerous works, I early in January transmitted to him a rough copy of my intended address, with a request that he would kindly point out any omissions. Although himself too ill to reply, he, by his friend Lieut. Safford, expressed his heartfelt gratitude for this recognition of his labours, while at the same time he confirmed the general accuracy and completeness of my proposed account of his researches."
The financial statement of the Society having been printed and distributed,
a discussion arose on the mode of keeping and publishing the Treasurer's accounts, which, having been brought to a close, the Rev. C. Pritchard proceeded to read the Report of the Council to the forty-sixth annual meeting. Of this comprehensive and elaborate document it is obvious our space will only allow us to give a summary and some extracts.
After stating that the number of members is now 511, being a small increase during the year, the Report continues as follows:
“ The Council cannot recollect any former occasion on which there has been better ground for congratulation to the Royal Astronomical Society than at the close of the past year. Looking backwards ten years, they find the number of the contributing
members has increased by nearly thirty per cent. The attendance at the Evening Meetings has more than doubled, and the funded property of the Society, during the nine years' tenure of office by the present Treasurer, has increased by upwards of 2,700l. stock.
“ Time, however, as upon all else, so upon us, has left its inevitable mark; and a Society which celebrates, as we do to-day, its forty-sixth anniversary, can scarcely hope to find many of the cherished names of its original founders still remaining on its lists. Of those eminent and energetic men who met together the
germ of the Royal Astronomical Society, on the 12th of January 1820, three only now survive; but it is worthy of our most grateful observation, that in Mr. Babbage, Sir John Herschel, and Sir James South (for these are the three), we recognise names which are among the most honoured that have adorned our annals.
“During the past year the Society has to regret the loss by death of more than an average number of long-valued and distinguished members, some of whom will begreatly missed, not alone by our own body, but by the whole commonwealth of Science. *
Proceedings of the various Observatories. At Greenwich the usual meridian observations have been continued; those of the asteroids being made in cooperation with the Paris Observatory, by which the labour is divided. The moon is observed with the albazimuth off the meridian at all possible opportunities. The stars contained in Bessel's Fundamenta are being re-observed. The new value of the solar parallax is used in the computations, and photographic registration of magnetic earth currents has been commenced.
At Oxford 1,043 transits have been taken, and the heliometer used in re-measuring Struve's brightest double stars. Meteorological observations have been kept up, and other work has also been done. The Cambridge Observatory is occupied with observing stars of large proper inotion-stars in Argelander's standard list, with solar and cometic observations and occultations. The Astronomer Royal of Scotland has been absent on his exploration of the Egyptian
pyramids during much of last year, but the zonal work of the Edinburgh Observatory has been regularly carried on. The interesting results of Professor Piazzi Smyth's measurements and photographs are not yet published.
At Kew 272 sun pictures have been taken ; arrangements made for testing sextants and pendulum experiments in connexion with the Indian survey have been executed, besides much important work in regard to solar physics. The Liverpool Observatory being in a transition state, only the distri
* We shall recur to this subject in our next.