413.—Photographic Apparatus in Connection with the Theodolite.—The application of photographic apparatus as an accessory to surveying instruments has been tried tentatively for many years. A practical introduction to the subject was first given by M. Laussedat in a paper published in the Comptes Rendus de l'Academie des Sciences, 1859. The subject has since been well studied by many writers, and is written up extensively by Dr. E. Deville, LL.D., Surveyor-General of Canada, in a work entitled Photographic Surveying, published in Ottawa, to which we must refer the reader for full discussion of the subject. In England, Mr. J. Bridges Lee has invented a very suitable camera in which a negative glass photograph of 4½ × 3½ inches is taken, with an axis line from the shadow of a hair permanently photographed coincident with the axis to the telescope as it appears to view. At the same time degrees and subdivisions are taken on the photograph to right and left of the axial line. The edge of the magnetic circle is also photographed upon the plate, indicating clearly the bearing of the station taken by the axis line. The whole of these operations are performed at once in a perfect manner.
414.—Mr. J. Bridges Lee's photo-theodolite was made in excellent workmanship by Messrs. Troughton & Simms. The inventor has published a paper on the subject, to be had of the Society of Engineers, Westminster.
Fig. 168.—Light camera upon the telescope of a theodolite.
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At the present time a camera is very commonly taken by a civil engineer for prospecting in new countries,—a convenient form of this will be discussed at nearly the end of this work—but it is not generally held that photography will ever offer a means of expeditious surveying, except possibly in very mountainous countries where the necessary stations for observation become difficult of approach and of clear definition. The objections to the more general adoption of photography are, otherwise, that the processes are in degree tedious, and require special skill in manipulation, and that the apparatus is heavy and expensive with sensitive glass plates for use with it.
415.—There are many cases, no doubt, where a photograph would be valuable for the exact definition of a station. To meet this case the author has made a small light camera, shown Fig. 168, giving photographs 2 × 2 inches only, with axis line from shadow of a point. The camera to be placed when required upon the telescope of a theodolite for special cases. He has lately used his patent slide for this camera that carries films which will be further described at the end of this work. The films are unbreakable, and remain sensitive many years if kept dry. The weight of this camera with its double slides and 100 films is about 1 lb. There is ample room for it in the ordinary theodolite case.