4 



SCIENTIFIC NEWS. 



[July 6, 1888. 



The centre of motion of the telescope or intersection 

 of the polar and declination axis is 37 feet 10 inejhes 

 above the masonry foundation. The sight line of j the 

 telescope is 5 j feet from the centre of motion, and the 

 end of the rod for counterpoising the tube 12 feet. 



The tube is made of hard steel plates rivetted together. 

 It was shipped in four sections (besides the cast-iron 

 central section), which are connected by bolts through 

 flanges at their extremities. The plates near the middle 

 of the tube are A inch thick, and the thickness of the 

 sheets diminishes towards the ends, where it is •£■ inch. 

 The tube is 52 feet long, 4 feet in diameter in the middle, 

 and tapers to a little over 3 feet at the ends. In the 

 shops of the makers it was tested by placing a ton on 

 each end when supported in the middle, and in other 

 ways, the greatest deflection produced being about one- 

 eighth of an inch. 



The object-glass by Alvan Clark and Sons is secured 

 to a flange on the outer end of the tube in the usual 

 manner. Its clear aperture is 36 inches, and the distance 

 of the focal plane from the back surface of the flint lens 

 is 56 feet. The lenses are 6| inches apart, and the total 

 thickness of glass, traversed by a ray of light, is about 2^ 

 inches. The weight of the objective in its cell is 



53° lb - 



The tail-piece at the eye end of the telescope is sur- 

 rounded by a revolving jacket, provided with position 

 circle, clamp, and slow-motion screws, for carrying the 

 spectroscope and other accessory instruments. Clamps 

 on opposite sides of the jacket receive two hollow brass 

 rods 6 feet long and 3 inches in diameter, and any 

 apparatus attached to these can be rotated easily and yet 

 firmly about the axis of the telescope. 



The draw tube at the eye end is 8 inches in diameter 

 and is focussed by a wheel surrounding and concentric 

 with the tube. This wheel acts upon three screws, 

 parallel to the telescope axis, which move the draw tube 

 in or out, and allow the heavy micrometer or other instru- 

 ment to be adjusted to the proper focus with great ease 

 and accuracy. The eye end is surrounded by a steel ring 

 39 inches in diameter to which lead all the clamps, slow 

 motions, and other contrivances operated by the observer. 

 The spokes of the right ascension wheels are notched, so 

 that they can be distinguished from the declination wheels 

 in the dark. 



There are three finders of 2f , 4, and 6 inches aperture, 

 and in addition to these, brackets to which the ob- 

 jective and eye end of the 12-inch equatorial can be 

 attached when a finder of great power is desired. The 

 makers are providing a double slide micrometer eye- 

 piece for this or the 6-in. finder, which will enable the 

 great telescope to be pointed at a faint object by means 

 of any neighbouring bright star — a contrivance especially 

 valuable for photographic work. 



The three microscopes for reading the finely-divided 

 circles from the eye end (two for declination and one for 

 right ascension) also pass through this ring. By turn- 

 ing a switch close to the eye-piece of the corresponding 

 microscope, the circle to be read is illuminated by 

 an incandescent electric lamp. Attached to the ring 

 are also a small sidereal clock, a telegraph key for record- 

 ing the time of an observation, and an electric switch 

 for starting or stopping the driving clock. 



The driving clock in the top section of the pier is, on 

 a large scale, essentially the same as the clocks employed 

 by Messrs. Warner and Swasey on their smaller equa- 

 torials and chronographs, except that it has an electric 



control, by which its rate is kept in agreement with that 

 of a standard astronomical clock. 



The equipment for photographic work is very com- 

 plete. The photographic corrector is a meniscus of 

 crown glass, 33 inches in clear aperture, and weighing in 

 its cell 150 lb. When in use it is placed in front of the 

 visual objective, and the focus of the combination thus 

 formed is about 10 feet above the eye end. At this point 

 a large aperture is cut in the telescope tube, giving 

 access to a plate-holder capable of taking a dry plate 

 20 inches square, or any smaller size, and provided with 

 all the necessary adjustments. An image of the moon 

 formed here is about 5I ins. in diameter. Instead of a 

 dry plate, a board holding an enlarging lens can be in- 

 serted in the plate-holder, and a magnified image of a 

 planet projected into a small box camera, screwed to the 

 draw tube at the eye end. 



The steel dome, 75 feet 4. inches in diameter, was 

 made by the Union Iron Works of San Francisco. The 

 weight of its moving parts is 100 tons. It is rotated on 

 the plan devised by Captain Floyd and Mr. Fraser, by 

 an endless wire rope, which passes round the cir- 

 cumference of the dome, over grinding pulleys, and 

 around a grooved wheel turned by a hydraulic motor 

 in the basement. The dome can be turned com- 

 pletely around in nine minutes. 



The slit for observing is 9! feet wide. It is closed by 

 two steel shutters weighing 15 tons, which are opened 

 by an endless rope hanging inside the upper gallery. A 

 pull of 5 lb. is sufficient to move the shutters. 



The hydraulic elevating floor weighs 26 tons, is 61J 

 feet in diameter, and is movable between fixed galleries 

 through a range of 16J feet. It is operated by four 

 telescopic hydraulic rams, which have replaced the 

 motors formerly employed for this purpose, their motion 

 having been found inconveniently slow. The motors are 

 retained, however, and can be connected in place of the 

 rams whenever desired. By means of the rams the 

 floor can be raised in a little less than ten minutes, and 

 lowered in four, with an expenditure of 300 gallons of 

 water. The floor is counterpoised by eight heavy 

 blocks of iron, which slide in vertical columns and re- 

 lieve the rams of all but two tons of the weight to be 

 lifted. 



PHOTOGRAPHY BY NIGHT WITH 

 "LIGHTNING POWDER." 



VARIOUS scientific journals and the special photo- 

 graphic organs have published formulae for a 

 mixture of magnesium and chlorate of potash, known as 

 lightning powder, by means of which photographs may 

 be taken in the night. We give, from La Nature, par- 

 ticulars of an improved process and apparatus devised 

 by Mr. T. Sardnal, which prevents all risk of accidents, 

 and obviates the escape of fumes from the burning 

 powder. The inventor converts the powder into pastilles, 

 which are then burnt in the apparatus shown in figs. 1 

 and 2. 



The powder used is that of Dr. Fabre, and consists 

 of:— 



Pulverised chlorate of potash, very dry . . 6 parts 

 Sulphuret of antimony . . . . . . 1 part 



Magnesium powder . . . . 3 parts 



When these three materials are weighed out, the 

 chlorate of potash and the sulphuret of antimony are 

 very carefully mixed together, using for this purpose a 



