He now resolved to try the experiment of applying photography to the determination of stellar parallax.
The adoption, by Sir William Huggins in 1876, of gelatine or dry plates in celestial photography was a change of decisive import.
The advances in stellar photography made by Paul and Prosper Henry and others suggested to him the magnificent idea of obtaining, through the collaboration of astronomers in all parts of the world, an autographic picture of the entire sphere containing more than fifty million stars, which should faithfully record in future ages the state of the sky at the end of the i 9th century.
For ordinary photography, however, there is this disadvantage: the image on the focussing-screen and the correct adjustment of the photographic sensitive plate are not in register; in astronomical photography this difference is constant, but in other kinds it depends on the distance of the objects.
In recent times photography has been so successfully applied to the mapping of our satellites as nearly to supersede visual observation.
By the aid of photography it is easy to prepare a plate, transparent where the zones of odd order fall, and opaque where those of even order fall.
Topographical surveys are gradually extending, and explorers of recent years are better trained for their work than they were a generation ago, whilst technical processes of recent invention - such as lithography, photography and heliogravure - facilitate or expedite the completion of his task.
Although some lurking errors impaired the authority of the concluded parallaxes this work ranks as a valuable contribution to astronomy, since it showed the possibility of employing photography in such delicate investigations.
The great development of photography has been a notable aid to explorers, not only by placing at their disposal a faithful and ready means of recording the features of a country and the types of inhabitants, but by supplying a method of quick and accurate topographical surveying.
He did much to advance stellar photography and its use in cataloguing the stars, and he was responsible for the geodetic surveys of Natal and Cape Colony, British Bechuanaland, German S.-W.