July 20, 1896.

THE AUSTRALIAN PHOTOGRAPHIC JOURNAL.

161

DESIGN FOU A PHOTOGRAPHIC TRANSIT CIRCLE.

By H. C. Russell B.A. C.M.G., F.R.S., Government Astronomer, New South Wales.

(Paper read before the Australasian Association for the Advancement of Science.)

Adjustment of Camera.

The provision for holding the photographic plate is similar to that used in the star camera, which holds the plate firmly against carefully made stops, so that it cannot move from the position in which it is held. Means of adjustment of this plate-holder would be provided, and also the means for fixing it rigidly when it is adjusted.

We now proceed to test the collimation of the camera, and we assume that the photographic lens has been set as truly as possible at right angles to its tube, and therefore, since it turns on the same axes as the telescope which has been adjusted in collimation, it will in the two positions of the instrument record the star on exactly the same point of the plate, except there be any unknown errors of collimation or flexure due to the position of the telescope in declination, and differing from those found to exist in the horizontal and vertical positions in which it was tested. During the time the plate was exposed to the star the observer has constantly observed the star on the wire, and if the bisection has not been perfect he has instantly corrected it. So that the mean position is one of accurate bisection of the star, and experience shows that this is possible, because perfectly round photographic star images are easily obtained, and it is obvious that such a bisection of the star is immeasurably better than that of the flying shot method unavoidable in the ordinary transit circle.

If the two images are not superposed then the centre of the spot made by the two images may be safely assumed to be the mean of the unknown errors, and if it does not wholly eliminate them it must at least be a more accurate position for the star than that obtained in an ordinary transit circle, which does not permit of the determination of these errors by any direct method, except in so far as the reversible transit circle of modern dimensions provides for it. But the star camera form of reverser works much more smoothly than the other form, and has the great advantage of reversing not only the telescope but also the stand itself, and at the same time avoids jars to the instrument, and provides for a far more accurate bisection of the star as we have seen.

In taking observations or rather photographs by this instrument two observers are necessary, and the work would be better and much more rapidly done by three--one to observe the star bisected by the wires, another to read the microscopes on each declination circle in each position of the polar axis, and the third to read and

record on a chronograph the time of transit. It has already, been mentioned that the graduations for 15 minutes of time on each side of the zero, i.e., the point indicating that the telescope was in the plane of the meridian, and the determination of that point by observation of a mercury reflector, have also been stated. It remains now to mention that RA graduations or marks should be at 10 seco nds intervals; this is quite possible since the RA wheel is 3 feet in diameter. Hence seven of these marks would pass under the miscroscope each minute; the microscope is powerful enough to enable the observer to see easily J of a second of time, and the parallel wires in the microscope should be exactly suited to the division lines. .A point on the graduations would be chosen so that the observation begun there would be soon enough to admit of reversal and picking up the corresponding divisions on the other side of the meridian. Since there would only be a few of-these division lines in use, it would be possible to determine with great accuracy their relation to the meridian line.

In making the observation the telescope would be set on the desired star before the meridian; the RA observer would give the signal when the plate should be uncovered and then record on the chronograph the time that each division of the circle passed the microscope. When done he would give the signal for covering the plate and its reversal, and then repeat the observation in that position as before. Now, since the graduated circle would move steadily on without the slightest vibration even under the high power microscope, he would be able to determine the RA with much greater accuracy than is possible in the ordinary transit circle, and that motion being always at the same rate, even fcr stars close to ihe pole, the observer would be able to record the RA with extreme accuracy for stars in all declinations.

I have, I think, said sufficient to place before you a fair outline of the instrument and the method of using it; to make this complete in every detail would require a small volume, and is not necessary. If I have demonstrated the principles involved, and I think I have done to, my present purpose will be served, and if my estimate of the quality of the work which would be done with this instrument is correct, the design for it will be all the better for discussion.

I would only add that the photographic field might be two degrees, and that there would be two images of any stars, save the guide star, that might be in the photographic field, and their positions relative to the guide star would be determined with the needful accuracy by measurement. With the 13-1 inches objectives used for the star chart, we can easily get a hard round image of an eleventh magnitude star in one minute with increased experience in manufacture ; and with Jena glass it is fair to assume that star images would be smaller and better defined, and measurement more accurate than it is now and even now they can be made good enough to justify the making of the photographic transit circle here described.

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