December, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



301 



mcnt with other reagents is also reversible, as hydro- 

 quinone has marked peculiarities. 



Mr. Sheppard's reference to the use of the word 

 "development" by Messrs. C. W. Piper and D. J. 

 Carnegie is beside the mark. This word is in common 

 use and has many applications. We talk of " de- 

 veloping " a carbon print when we merely wash away 

 the soluble gelatine from it with water; we " develop 

 a platinum print when we bring about a simple chemi- 

 cal reaction; an essay writer " develops " his subject; 

 and an athlete " develops " his muscles. In the " re- 

 development " referred to there does not appear to be 

 any distinction between developable and non-develop- 

 able silver salt — it seems to be rather a simple case of 

 removing the halogen from its silver compound. 



A yew Tripod. — The limitations of the ordinary 

 tripod are all too well known bv those whose work re- 

 quires the camera to be supported in other than an 

 approximately horizontal position or in places where 

 the ground is steep and irregular. These difficulties 

 are practically overcome in Butler's " Swingcam " 

 camera stand. The wonderful adjustability of this 

 stand is obtained by attaching each leg to the tripod 

 top by means of side links and a swivel block. In re- 

 placing a single movement by three separate move- 

 ments a loss of rigidity might be expected, but the ad- 

 justing screws so clamp the whole of the moveable 

 parts together that the maker claims an increase rather 

 than a loss of stability. Further conveniences in con- 

 nection with the stand are extending bars which will 

 raise its height to over seven feet, and swivel points to 

 the legs, so that the points may be fixed vertically to 

 obviate slipping when the legs are unduly splayed out. 

 These points may even be bent up and converted into 

 hooks, so that, for example, one leg may be secured to 

 a convenient object above the camera, such as the 

 branch of a tree. It is impossible to describe all the 

 ways in which the stand may be u.sed. 



The Thornton-Pickard Company send us tlie list of winners of 

 the /^loo Competition for pictures taken with their cameras. The 

 numerous prizes range from £\o to £1. 



ASTRONOMICAL. 



By Charles P. Butler, A.R.C.Sc. (Lond.), F.R.P.S. 



The Newly Mounted Three-Foot 

 Crossley Reflector. 



This fine instrument, which was first erected at the Lick 

 Observatory in 1895, has been almost entirely remodelled 

 according to designs of the astronomers there. Although 

 much excellent work was done with it in its original condition, 

 it was found that many details were unsatisfactory for the 

 prosecution of the delicate investigations which the late 

 Director Keeler had proposed to devote its great light- 

 gathering power. From the extremely promising records 

 obtained it was thought that the great value of its excellent 

 parabolic mirror in astronomical photography emphasised the 

 desirability of supplying it with a more rigid and efficient 

 mounting than the one comprised in the original bequest. 

 This was authorised by the Board of Regents in 1902, and 



after much consideration it was decided to adopt the form 

 shown in the illustration. Tiie heavy parts of the mounting 

 were built by the Fulton Engine Works of Los Angeles, while 

 the driving clock, slow motions, guiding mechanism, and all 

 small parts were constructed in the Lick Observatory repair- 

 ing shops. 



The polar axis is of heavy boiler steel, furnished with steel 

 jackets shrunk on to form the bearing surfaces. The declina- 

 tion' axis is a heavy shaft of solid steel passing through the 

 centre of the polar axis. One end carries the tube of the tele- 

 scope, the other the usual counter-balancing weights. 



The tube consists of a cubical central section of castings, 

 with cylindrical sections above and below the cube to carry 

 the eyepiece and large mirror respectively. The upper end 

 section can be rotated for adjustments in position angle. 

 The total weight of the moving parts is about 6 tons, and 

 although somewhat cumbersome has the important advan- 

 tage that it allows of passing the meridian in all positions 

 without reversal. 



As the driving mechanism had to be very delicate, the 

 driving sector was made large, with radius of eight feet, the 

 worm having a pitch of one-tenth of an inch, and as a com- 

 plete driving wheel of this size would have been too unwieldy, 

 two sectors were provided, so that one could always be ready 

 for gearing up as soon as the other was running out. Each 

 sector will run for 66 minutes. 



In connection with the optical arrangements important 

 modifications have been introduced. The ordinary method with 

 Newtonian reflectors of throwing the image out at the side of 

 the tube by a plane mirror placed diagonally has been dis- 

 carded, and the plate holder fixed in the centre of the tube so 

 that the plate is in the direct optical axis of the large parabolic 

 mirror, and of course it cuts off in this position no more, or 

 probably less, light than did the original diagonal mirror. 



The adoption of this system then rendered it necessary to 

 bring the screw adjustments of the plate holder to the side of 

 the tube, and this has been done along one of the thin plates 

 which hold the plate carrier. By means of a small reflecting 

 prism the guiding star at the side of the photographic plate is 

 also viewed by a subsidiary telescope at the side of the tube. 



On account of the old difficulties special precautions were 

 taken in designing the mirror cell and supports, and it is very 

 satisfactory to find that Professor Perrine reports that the 

 whole is very satisfactory after several months of use. The focus 

 of the instrument can be determined easily to 0-005 inch, and 

 it does not appear to change greatly with variations of tem- 

 perature. No change of focus has yet been detected in a 

 range of less than 10 ' F. Evidence of the great efficiency of 

 the instrument is afforded by the splendid success achieved by 

 Professor Perrine in his recent important discoveries of the 

 VI. and VII. satellites of Jupiter in the earlier part of the 

 present year. 



