This instrument was erected at Cape Ledo, close to a hill of such inclination that the sun could be followed during the whole of the eclipse, while the long tube could be manipulated with greater advantage than would have been possible if the instrument had been erected on level ground. The hot air rising from the heated hill probably affected the definition in the photographs, but under the circumstances that could scarcely be avoided.
This form of mounting certainly solved the question of the possibility of using long-focus lenses mounted as direct photo-heliographs, but the apparatus is certainly unwieldly, and was only got into the fit state that it was on the eclipse day by the very great care and patience of Professor Bigelow. As it was intended principally to photograph the partial phases of the eclipse with this instrument, instantaneous exposures were arranged for, but Professor Bigelow succeeded so well in the adjustment of the instrument and the regulation of the sand clock, that he would have tried to obtain photographs of the lower corona with it had the weather permitted.
The photographic apparatus on this instrument has a very ingeniously constructed revolving plate holder, carrying round plates of twenty-two inches diameter. The exposing apparatus and the apparatus for rotating the plate between the exposures were moved by pneumatic arrangements, exposures being made at intervals of six seconds, the exact time of each being recorded on a chronograph. As no dark slides were used, it was necessary to enclose the whole of the photographic apparatus in a dark-room. One hundred and ten exposures were made with this telescope during the partial phases of the eclipse, all the photographs taken having to be obtained through clouds.
In several expeditions previous to this, where more than one kind of observation has been required, two or more objectives have been mounted on the same stand and driven by the same clock; but this plan is always open to the objection that any accidental disturbance in the manipulation of one of the pieces of apparatus will most probably spoil the results for both. With the American plan of many objectives on one heavy axis, and a pneumatic apparatus to manage all the actual operations of exposures and changing of plates, this objection of possible accidental disturbance is to some extent overcome; but the shake of the many operations taking place on the one axis introduces another risk. Beside this, the apparatus is very heavy, and exceedingly difficult to transport and erect, even in a civilized country.
Such is a very summarized account of the instruments hitherto employed, and it seems to me that the time has now come when much can be gained by the employment of fixed instruments and a moving large plane mirror. This idea of using a heliostat is, of course, not new, for it has been used several times on a small scale, and for special purposes. There is nothing beyond the difficulty of making a plane mirror sufficiently large for the work to prevent the adoption of this method in the future; and this difficulty now has ceased, as it is only a matter of time and labor to make plane mirrors of sufficient size. With a large plane mirror, twenty inches or upwards in size, mounted on a heliostat mounting, and so arranged as to reflect sunlight into a series of instruments rigidly supported in a horizontal position, the difficulties of eclipse observers will be very considerably lessened. The one driving clock will keep the pencil of light constantly in the same direction, and this can be used partly for photographing the corona, partly for spectroscopic work, partly for polariscopic observations, and so on for any other purposes, the whole of the instruments being fixed in the best possible positions for the observers. Practically, with a large flat mounted in the manner indicated, we can fix any portion of the sky we require to observe, and to do it we can point as many instruments as we can crowd into the pencil, each instrument being quite independent of the others. The length of focus of an objective would not introduce any difficulties on this plan, for the length of the tube is of little importance when it can be fixed in an horizontal position. The observers at the Eclipse Station only have one astronomical adjustment to make, i.e., that of the position of the heliostat, and only one driving clock to regulate. This clock, since it has only to move the weight of the plane mirror and its mounting, can be more accurately made and regulated than is possible with a clock when it has to carry the weight of the tube and heavy axis of an ordinary telescope. The positions of the observers are more easy and natural during the precious seconds of totality; or, if personal superintendence is to be abolished in favor of the American pneumatic apparatus, this suggested arrangement of the instrument is better fitted for the pneumatic attachments than the old plan is. The whole of the photographic apparatus can be fixed up in a dark hut or under a dark tent with far less trouble and risk of stray light than is possible with the old manner of mounting.
The cost of a good heliostat mounting is about the same as that of a good telescope, and with one heliostat we can do the work of at least half a dozen of the usual instruments.
A. A. Common, F.R.S., and A. Taylor, A.R.S.M.
The Supreme Court at St. Paul, Minnesota, handed down a decision on July 1st in the case of Ida Moore, of Minneapolis, against Photographer Rugg. Rugg sold a copy of Mrs. Moore’s picture, which was put on exhibition in improper places, much to the discredit of the lady, and she brought suit for damages. The Supreme Court holds that it is a case in which there is ground for the recovery of damages; that the photographer has no right to dispose of pictures which are the sole property of the sitter. The decision is an important one. Similar cases have arisen once or twice previously in other parts of the country.