Photography played an enormous part daring the Great War. Airplanes Wwere provided with photographie eyes Which could record instantaneously ind accurately the minutest detail of the enemy lines from the great height two or three miles at which they Jhad to fly 40 be out of range of anti aircraft guns. Before an_ attack, officers were provided with maps cor~ Feeted to the last minute as a result of the use of panchromatic plates and filters transmitting only the haze penetrating rays. In 1918, the Britis JSrmy serial observers took over a Quarter of a million "negatives and fade nearly six million prints from them, The best efforts of camouflage artists were of little avail against the eye of the camera, although to the haked eye the painted canvas was ult indistinguishable from the green field or the dappled woodland, Peace Research, The fruits of war have been reaped in time of peace. The experience gained under stress of necessity has Been turned to the innocent surveying of uncharted country, and to the cor= rection of maps already surveyed Taboriously on land. A map of a city made by photography from the air ina few hours would take many months to complete by the ordinary methods of surveying. An accurate Survey of the distribution of cars Farked at any moment in a large own would be quite impossible unless it-was made from the air. This was recently successfully done at Washington in connection wilh a trame survey, made by Dr. Miller MeClinfock of Harvard University. The Whole" city" was surveyed in 35 minutes.
The value of photography tes Us ability to obtain a highly _accurate record of events which cannot be seen to the eye, or which change
corded by the pencil, Not only does Faplaly that they cannot. be re: the plate see things by light to which the eye 1a not sensitive, but it is able impressions of things which feeble to be seen by the eye, rapidly as to quite invisible. "The plate "has the Power to build up faint impressions Until they are sufficiently intense to be seen. This property is quite foreign to the eye. It has enabled the astronomer to record thousands. of its which would otherwise have gone unobserved.
Bullets It has, moreover, the arrest the motion of the ject. Photographs from a revolver are just as cleat if the projectile were ied ta Hlonary in front of the lens. The camera enabled the timing of the Schneider Trophy air 4 the Olympic sprints to be made with fan accuracy of one-hundredth of second.
Explosions happen quickly. Tt does not take long for" flame to pass through an explosive mixture "of tasses ina coal-mine shaft, leaving Geath and destruction in ite wok Here again photograph's ald is sought. An explosion made 10 occur in-a long lass tube is photographed on a film Wrapped around a drum revolving onan axis parallel to the tube. A Streak ie recorded on the Alm. and from its slope the speed of the ex. pletion is deduced. "Data derived in {his way has proved of considerable aug to thave concernned with safety in mines.
All are familiar with the carbon are used as" the source of intense sector. ton showed that, light light In the motion-picture-film pro-
from such a source, It passed through 8 prism, was spread out into a jegirum having the colours of the rainbow. Ifa trace of iron is put on one of the carbons and the ight fg made to pass through a narrow slit before falling on the prism—as in the instrument. known as. the spectrograph—ihe spectrum obtained 4 full of bright lines which can be photographed. Their position is fixed, and if they are ever observed spectrum they are known to have been due to iron. In the same way the presence of other substances may be detected and their amounts sured, This method—of "Spectrographic Analysis" has proved of me elimable service to the chemist. Tt provides him with a method of analysis a million times more delicate than the most refined chemical means. It has enabled the astronomer to identity the elements that are present in the bodies of the sun and the stars, and to ascertain the composition of their atmospheres. By using the spectrograph, the physicist was able to supply. substantial verification of Einstein's theory of relativity.
Photography is truly the servant of mankind, "Science technology, and human Welfare owe ita debt of incalculable magnitude. Its services.1o mankind. are enormous.
Text read from the printed page and cleaned; old type leaves the occasional error.