THE DAY BEFORE

Evening Post, Volume CIV, Issue 91, 14 October 1922, Page 11

THE DAY BEFORE

AWAITING THE ECLIPSE

INSTRUMENTS AT WALLAH

WELL-EQUIPPED EXPEDITION.

(By Dr. C. E. Adams, N.Z. Government

Astronomer.)

No. 3.

. WALLAL, 20th September.; At Wallal the camp is laid out in an East-West direction, and from the arriTal there to the day of the «olipse irark was carried on by day and by night. IS was necessary to put in concrete founds,-, tions for all.the telescopes, cameras, and coelostats. Fortunately good sand and rook were available, so that rapid progress was made. The Lick Observatory party is located ■at the east end of the camp. The principal instruments are ihe 40ft camera, the two Einstein cameras, the five spectrographs, and some smaller cameras.. Of these the 40ft camera and the wooden, tower dominate the camp. This is the longest telescope that has been used for photographing the corona. Most cameras of long focal length are mounted horizontally or vertically, and the sun's light is then reflected by a mirror into the tube. It has been the practice of tho Lick Observatory to build the 40fb camera pointing directly to the position the sun will be in at the eclipse. The camera is made up at the camp from lengths of gas pipe, and is about Ift square at the lens end, widening out to 4ft square at the other. The whole tuba is covered with black cloth, and mad» light-tight. To avoid any vibration in a camera of this' size a number of precautions are necessary. The lens is sin aperture and is supported on a wooden tower about 36 feet high. The tube does' not rest on the same tower, but is supported by art external wooden tower. 1 On the day of the eclipse, the outer tower is .covered; with canvas to stop any vibration of tho inner tower At the lower end the plateholder is supported on' concrete blocks, and on them is placed the metal tract along which the plate-holder moves. In a camera, of this length the tube remains fixed while the plate-holder moves by clockwork, at tho same rate as the.-sun's imags moves. In this way it is possibls to make exposures of considerable length.. A light-proof tent is built over the plateholder, and in this tent the observers work. The. camera, gives an image of ■the sun about 4£ inches in diameter, and this image is in full vi^r on the plate irhile the photograph is being taken. Th» chief purpose of this camera is to tak» large-size photographs of the corona during the total phase of the eclipse. Th« plates are 14 x 17 inches, and during ths preceding days constant drills are held, so that in the darkness of the eclipse th« plates can be handled rapidly, and tht various exposures made. . '

THE EINSTEIN CAMERAS.

The Einstein cameras are placed nerf the tower telescop*. These cameras wer« specially constructed for this expedition.. There are four of these cameras, two of 15ft focal length and two of sft. The lenses are photographic doublets designed to give a flat field over the larg« plates. The camera frames and solar axes are made of rigidly braced iron.. The plates are 17 inches square, made of plate glass.half an inch thick.

Full provision is made for speetroscopia observations of the light from the corona by mounting a battery of five spectrographs on a wooden polar axis. All the polar axes are on ball-hearmgs on substantial wood beams bolted * to concrete blocks, and canvas sun, and wind shelters are erected round tho cameras. Everyone at the camp has been busy with this work, which does not cease at sundown, as much adjustment has to be done a,t night, *nd check photographs taken. The Canadian party is located to th«f South of the lick party. The chief instrument is a photogra-phio telescope ol lift focal length- with a specially designed doublet lens for photographs of the sun and stars, to test the Emsteintheory. This ■ party { is also using * polariscope on the corona.

Mr. J. Evershed has erected his instruments to the west of the camp. Ho' has a 20ft came*a mounted horizontally, with a 12in lens and a- 16in coelostat, as well as a number of spectrographs ; some of thesa are of his own design,, and are of very wide dispersion. Mr. Hargreaves and Mr. Maxwell havea. magnetic declination instrument of tha British'meteorological type. They have the Cortie 4in lens of 19ft focal length mounted horizontally with an Bin coelostat for photometric measurements of the light from the corona. A Cooke lens of 8j inches focus, and working aH F2, is intended for the direct photography of the corona, using various plates and light screens. / ■ The Commonwealth' Meteorologist supplied a number of thermometers, a wind gauge, and other instruments, as well aa quick-running barograph ' and thermograph. •

It will be seen that the expedition ia very well equipped, 1 and that.the whole camp is kept busy getting instruments erected and tested. ■ ACCURATE PREPARATIONS. ' The first observation on ajrival was to determine the direction of the north and south line by astronomical observation, and to this datum line all the instruments have to be adjusted.

The dust nuisance makes all delicate work very difficult, and photographic work is carried out under extremely unfavourable conditions. The air is frequently nearly saturated with moisture,. and this makes plate-drying.a very long operation.

Two kinematographers are with, the expedition. They keep a pictoral.record of all the camp "activities, and are preparing to take a film of.the eclipse. The drills at the different instruments are most complete. One of the naval men, with a good clear voice,, is stationed in a convenient ' position with the chronometer in front of him. He calls out warnings ten and six minutes before the estimated time of totality, . when everyone takes up his station. Another warning is given at two minutes befora totality, when very careful watch is kept to see the precise instant the total phass begins. Immediately this occurs, the time-keeper calls out the seconds for the whole time of the eclipse, and each observer exposes the plates at the . pro-. ai-ranged times. '

The weather has kept uniformly fine, and no delays have occurred through having to wait for clear skies. Every day it has been possible, to observe the sun at the time of the eclipse, and to adjust the telescopes.

As some of the observations requira as accurate time as possible, a wireless station has been established at the camp. Wireless time signals have been received from Bordeaux, France; Annapolis, U.S.A.; ,aud from some of the Australian observatories. Time checks have also been obtained from the Perth Observatory over the telegraph line, and for two minutes every day the clock beats of the Perth clock have been received.

As the day of"the eclipse approaches, more steps aa'e taken to abate any posu sible dust -nuisance by spreading boughs of wattle round the camp and by keeping a team of natives carrying water from the well, and liberally sprinkling the ground round the camp.