May 28, 1896] 



NA TURE 



85 



•costly machiner)' in order that the camera shall follow the 

 apparent motion of the stars. 



The accompanying photographs were taken with a 3i-inch 

 refracting telescope of 29 inches focus, totally unprovided with 

 any driving mechanism, not even a tangent screw and slow- 

 motion rod, the guiding having been performed entirely by hand. 

 The correct rate of angular motion was secured by constant 

 visual observation of the image of a star, much out of focus, as 

 seen in a 2j-inch guiding telescope, carrying an eyepiece magni- 

 fying fifty times. This was mounted side by side with the 

 telescopic camera, and moved with it. 



Fig. I shows the instruments mounted on a firm equatorial 

 stand which is supported on a home-made brick pillar. The 

 2j-inch guiding telescope, by Cooke of York, is seen on the left, 

 provided with its total reflection prism and ej-epiece, and just 

 above it is a small " finder." On the extreme left is a counter- 

 poise which balances the 3J-inch photographic telescope, which 

 is on the opposite side of the declination axis, and is mounted 

 in a home-made wooden tube of square section, with dew-cap 

 and diaphragms of the same material. At the lower end the 

 dark slide is seen, and behind is a smaller camera which carries an 

 ordinarj' portrait lensofaj-inch aperture, which is used for obtain- 

 ing a duplicate photograph, on a smaller scale, simultaneously 

 with the larger one. The whole is so evenly balanced by the two 



:ope (as used for hand 



counterpoise weights — one of which is seen low down on the 

 right — that when undamped it remains at rest in any position. 

 Adjusting screws are provided in order to move the telescopic 

 camera slightly in right ascension or declination whilst the guid- 

 ing telescope remains stationary. This enables one to use the 

 nearest bright star for guiding purposes when the centre of the 

 photographic field contains no conspicuous star.s. Absolute 

 parallelism of the two telescopes is of no importance, but their 

 rate of angular motion must be identical. Interesting results can 

 be obtained with such a telescopic camera without any guiding 

 whatever. The camera remaining fixed, tlie images of the stars 

 travel along on the plate and leave " trails," which appear on the 

 negative as straight or curved parallel dark lines. 



By jilacing a small ink dot at one end i il each of these lines, the 

 relative positions of the stars can be indicated. It was found 

 that the faintest stars visible to the naked eye, leave trails on 

 negatives taken with such a 3i-inch camera, and accurate charts 

 of stars down to the sixth magnitude can be very easily secured 

 in this manner. 



These trails can also be usefully employed in certain cases to 

 secure records of the changes of brightness of " variable" stars, 

 as faint stars give ver}' fine lines, and briL;hter stars leave thicker 

 and denser ones on the negatives. Yariutions in brightness are 

 thus recorded in the varying thickness and density of the lines, 



XO. 1387, VOL. 54]. 



which are compared with the trails of other standard stars near. 

 From what has been said about trails, and seeing that the image 

 of a star moves more than its own diameter on a stationary plate 

 in a few seconds, it is evident that all the naked-eye stars can be 

 photographed with such an instrument with an exposure of a 

 few seconds. As an illustration of this, a photograph taken with 

 an exposure of only fifteen seconds, when the crescent moon was 



close to the Pleiades, showed not only the crescent, but also 

 the "old moon in the new moon's arms," due to earth-shine, 

 and twelve of the stars in the Pleiades. Accurate hand driving 

 for such a short period is a matter of comparative ease. 



Fig. 2 shows a photograph of Orion's belt taken with an ex- 

 posure of thirty minutes. The negative on close examination shows 

 stars down to the tenth magnitude. In the region represented. 



only about eight stars can usually be seen with the naked eye. The 

 photograph shows that amateurs can obtain, by half an hour's 

 exposure, a chart of any region of the sky, much more accurate 

 and revealing a far larger number of stars than are shown in the 

 star atlases usually in their hands. These phctographs, obtained 

 by such simple means, can always be used as records, and might 

 easily servefor the detection of "new" and "variable" stars 



