The Branches of Photography

Photomicrography.

The application of photography to microscopy covers a wide range. Small cameras are fitted to ordinary visual microscopes and used to make records of value chiefly for reference. In a more elaborate apparatus, a camera on a stand is swung over the vertical microscope, but for serious photomicrography, it is usual for the camera and microscope to be mounted on large stands or preferably on one rigid optical bench togethe r with the illuminating system.

Recently, special photomicrographic apparatus has been built, in which the ordinary microscope is replaced by a part of the op tical bench itself, such apparatus being particularly designed for the photomicrography of metal specimens.

For many years, the highest powers of the microscope were used only for the examination of bacteria and of test objects, such as the minute structure of diatoms, but the demands of the metallurgists and of research bacteriologists are now reaching to the utmost limit of the resolving power of the microscope.

Since resolving power is limited by the wave-length of the light used, photomicrography with ultra-violet light is now of value. It has been used with remarkable results in the fields of biology and is also being applied to the study of the minute structure of metallic alloys.

Apparatus for this purpose designed by A. Kohler was intro duced by Carl Zeiss in In the photography of stained specimens the control of the colour of the light is of great importance, and for this purpose sets of light filters have been designed by means of which stains can be photographed by light exactly complementary to that which they transmit. The maximum contrast is thus obtained.

Phototopography.

Col. Laussedat of the French army was the pioneer in the application of photography to surveying, and since photographic surveying is more suitable for mountainous countries, it was in the Alpine regions of Europe and the moun tain chains of Canada that the largest areas were first surveyed photographically. Dr. Deville, late surveyor-general of the Do minion of Canada, studied the subject very exhaustively and wrote a number of books on the subject, and the Russian surveyors have also used photographic methods on a large scale, especially in their surveys of Siberia.

In order to make a photographic survey, two photographs are taken of the same area from different known positions which are plotted on the map. Lines are then drawn from the positions to each identifiable point on the map and object in the photograph until the complete survey is accomplished, this plotting being done after the manner of a plane table survey but with the advantage that the detailed plotting is done in the office and is therefore independent of the weather, the only work done in the field being the photography. The cameras used for the purpose are of rigid

construction and have levelling mounts so that they can be levelled precisely. Elaborate photo-theodolites have been devised for the purpose, notably a beautiful instrument designed by Zeiss, but simpler cameras are also quite satisfactory provided that they are rigidly made. (See Plate III. fig. 4.) No considerable area can, of course, be mapped without a skeleton of positions determined relatively to each other with precision, and the primary triangu lation is therefore made over the area to be mapped, other points then being interpolated from the photographs.

Another method of photo-surveying is by the use of the prin ciple of stereoscopy. Two photo-theodolites are put at the oppo site ends of a measured base and exposures made in which the same objects are included. From the photographs distances can be measured by the use of the stereo-comparator designed and perfected by Prof. Pulfrich of the optical research establishment of Carl Zeiss at Jena. A notable advance in this field of stereo scopic photo-surveying was made when Captain von Orel of the Austrian army developed the stereo-autograph, which is a three-dimensional pantograph which can be used in conjunction with a stereo-comparator and will plot a map directly from the photographs. Stereo-photo-topography is now the most advanced of photographic methods of survey ; its accuracy is high, and it is very rapid. Its drawbacks are the high cost of the instruments, the necessity for base measurement, which is not always easy, and the considerable weight to be transported. (See Plate IV.) Aerial Photography.—Photography from the air is utilized for military purposes as well as for surveying. Its use during the World War was very extensive, thousands of negatives being taken every day by the armies. The first aeroplane cameras were modifications of standard cameras with usually a focal plane shutter, and the material was generally plates carried in holders and later in magazines adapted for quick changing. Hand operated cameras using magazines were later developed and were used largely in 1916 and 1917. Later, semi-automatic plate cam eras were designed in which the observer released the shutter while the mechanism at the same moment operated the changing of the plates. In the last years of the World War (1914-1918) these cameras were made of very considerable size as well as of very considerable weight, sometimes as many as so plates each of which measured 18 cm. X 24 cm. in size and were carried in a camera at one time.