Wallal Downs is not a town but a homestead
located on the north west coast of Western Australia
Some may believe that this deflection was already proven by measurements made at a total solar eclipse in 1919. However the results obtained at this eclipse were inconclusive and could just as easily have supported the value predicted by Newtonian mechanics, which is half that predicted by Einstein.
The eclipse at Wallal in 1922 was unusually long at 5 minutes and 18 seconds, and presented an excellent opportunity for the longer photographic exposures required by the less sensitive photographic emulsions available at that time.
The principal involved in the test was to take an exposure of the star field around the Sun at the time of total eclipse and to compare the stellar positions with those of an identical star field taken six months earlier in the night time when the Sun was not in the sky.
The foundation professor of Physics and Mathematics at the University of Western Australia, Alexander Ross, was present at the eclipse as a member of the Lick Observatory team from the USA. He wrote a small book in 1923 about Einstein's Theory of Relativity and included a description and photographs of the expedition. The title page is shown below:
The following are Professor Ross' words.
"On 21st September, 1922, the Sun was totally eclipsed by the Moon for a period of 5 minutes 18 seconds a Wallal on the North-West coast of Western Australia. The advantages of the locality for observations of the Einstein effect had been pointed out in a paper sent in 1920 to the Royal Astronomical Society by Mr R. D. Thompson and the author, while advance copies of the data had been forwarded to the leading observatories and to standing eclipse committees. Three expeditions finally proceeded to Wallal for the study of the Einstein effect. These were the Crocker Expedition of the Lick Observatory under Dr W. W. Campbell, a Canadian party under Professor Chan of Toronto, and an Indian party under Mr J. Evershed, of Kodaikanal Observatory. The Commonwealth Government acted as hosts for the visitors, and sent a naval detachment under Lieut-Commander Quick. The parties arrived at Wallal on 30th August, and proceeded at once with the work of erecting the instruments.
The Lick Observatory expedition had four special cameras - two of 5 ft and two of 15 ft focal length - built to Dr Campbell's own designs. The girder construction employed throughout gave a very great rigidity, which was a matter of the highest importance of cameras of such size. The lenses used were quadruplets, made according to specifications by Hastings and Ross, and gave wonderful definition over the large 17in x 17in plates. In the case of the 5 ft cameras, the focus did not vary by so much as 1-20th inch over the plates. In the case of one of the 15ft cameras, the variation was about 1-40th of an inch, and in the case of the other 15ft camera, it was too small to be detected. As these lenses gave fields of 15 and 5 degrees diameter respectively, the excellence of the workmanship was marvellous.
Everything was in readiness for the eventful day, and on 21st September the meteorological conditions were of the highest excellence. Photographs were obtained, which as already stated, led to a decisive vindication of Einstein's generalised theory of relativity. The star images were displaced in accordance with the law deduced from Einstein's theory, and the amount of the displacement differed by less than one percent - or within the limits of observational error - from the theoretical value. It is to be remembered that even the maximum possible deflection of 1.75 seconds of arc corresponds to a displacement of only 1-2000th of an inch on the plates of the 5ft cameras, or to 3-2000ths of an inch in the case of the 15ft camera. As the actual measurements had to be carried out on stars at some distance from the Sun's edge, the actual displacements of the observed stars ranged from about half down to less than one-tenth of the above amounts. These figures will indicate the extraordinary accuracy obtained in the investigations."
The following images are from those Ross provides in his booklet.
Unloading stores and scientific apparatus from the schooner Gwendoline at Wallal
Camp with donkey team, about a mile from the beach
A 15ft camera
One of the photographic plates
The 11ft camera used by the Canadian party