interpretation. At the best of times the vertical view presents all objects in an unfamiliar aspect, while in modern warfare the arts of camouflage are enlisted to render interpretation harder yet. In aerial photography the greatest foes to camouflage are stereoscopic pictures, and the fact that the photographic plate is differently sensitive to colors than is the human eye. Thus often gun coverings and concealed dugouts, not noticeable by the observer as he flies over, show clearly in the photograph he brings back, since the camouflage paint is a visual but not a photographic match with its surroundings. Camouflaging pigments had, therefore, to be tested photographically, and in turn plates and color filters were sought which would defeat the efforts of the enemy camoufleur.

The every day problem of the interpreter of photographs was to detect changes of any sort the substitution of artificial trees with concealed listening posts for real trees; the removal of sod to be used elsewhere for camouflage. For this purpose photographs made on different days were laboriously compared, side by side. Even when this was done, minute but important changes would be missed, a common failure which led to several proposals to facilitate such comparisons. One was the use of the "blink microscope " in which the two pictures were viewed successively in the same position, any change showing as a fluttering or blinking in the scene. In another ingenious scheme, adapted from the astronomical method of searching for moving asteroids, a positive made from one negative is laid over a negative of the same subject made at another time. If no change has taken place the two merge to a neutral gray. If anything in the view has moved, it stands out in striking contrast with the undisturbed parts.

In this brief sketch of war-time photography chief emphasis has been laid on the contribution of photography to the winning of the war. Reciprocally the demands of war have worked to advance to no inconsiderable degree the science of photography. This will be manifested, if in no other way, in the production of photographic apparatus of greater accuracy and reliability

of performance. The impetus given to research by the quest for emulsions of greater speed and sensitiveness has already resulted in unexpected progress, and this research may be relied upon to bring forth even greater improvements. The addition of an entire new department - aerial photography is undoubtedly the greatest advance due to the war. It opens up a new territory, and appears destined, quite apart from its wide pictorial uses, to enormous usefulness in mapping. It promises indeed quite to revolutionize our present methods of charting the earth's surface.

ΤΗ

VII

OPTICAL GLASS FOR WAR NEEDS

HARRISON E. HOWE

HE optical-glass problem, so far as the United States was concerned, can be simply stated. Large quantities of dependable quality were required immediately, the varieties being limited to a half dozen or so necessary for military optical instruments. It should be understood that by optical glass is meant that type of glass which is so made that its physical characteristics may be controlled within rather narrow limits, so that it is suitable for the exacting requirements of photographic lenses, range finders, spotting telescopes, binoculars, periscopes, gun sights, and similar modern warfare requisites.

In order that the complexity and magnitude of this problem may be more clearly understood, it will be well to examine briefly the history of its development elsewhere and understand the condition which prevailed in our country prior to August, 1914.

Prior to 1886 the glass makers were offering a very limited variety of optical glass to the makers of refracting instruments, and the perfection of the various microscopes, telescopes, etc., was necessarily limited to the possibilities presented a few crown and flint glasses. The possibility had been established of combining two lenses made from the available glasses into a doublet so as to bring pairs of colors to a common focus on the optical axis of the lens, thereby diminishing chromatic aberration. Means to render the image almost entirely free of spherical aberration had also been devised, but no attempts were

made to introduce new glass fluxes, and effort was expended only in perfecting technical manipulation and adding to the list of dense flints.

To this state of affairs there were, however, a few notable exceptions: Frauenhofer, the German optician; Faraday, the great investigator; and Harcourt, an English clergyman. Frauenhofer succeeded in finding glass which showed a diminution of the secondary spectrum, but the new glass was not produced on a commercial basis and the formula was unfortunately completely lost. In 1825 Faraday was appointed by the Royal Society, together with Sir John Herschel and Mr. Dolland, on a committee to examine, and if possible, to improve the manufacture of optical glass. The results of the systematic and very exhaustive experiments were reported minutely by Faraday in 1829, and although glass so found did not prove to be of important practical use, yet the work performed had much directional influence on subsequent researches.

Harcourt could not obtain from his small meltings pieces of sufficient size and perfection to permit a complete spectrometric analysis, and lacking information which could be gained only with the spectrometer, his subsequent work suffered for want of guiding experience. However, these researches were not entirely in vain, since certain facts were established relating to the effect of some chemical elements upon the refraction of light.

Until the late seventies silicon, sodium, potassium, calcium, lead, and oxygen had been the only elements used, excepting perhaps alumina and thallium in an experimental way. Crown and flint glasses were being produced of a far better quality as regards clearness, freedom of color, and homogeneity, and flint of far greater refractive power and dispersion, than had been offered up to this time..

In the late seventies Professor Ernest Abbe of the University of Jena published a paper on the microscope, in which he made an appeal to scientists to take up the improvement of optical glass, and pointed out that scientific instruments were in

a state of arrested development awaiting the perfection of glass which would offer a greater diversity in mean index, and mean dispersion, and render possible a higher degree of achromatism, thus diminishing the secondary spectrum. This plea attracted the attention of Otto Schott, and after communicating with Abbe, the two began an investigation of the problems, and started first of all to determine the chemical-physical principles underlying the making of optical glass. In experimenting with various combinations of elements new to the glass industry, several limitations had to be borne in mind. First, the flux must not act upon the material of the crucible and so absorb impurities. Second, elements which evaporate during the process tend to produce veins and must not be used. Third, cloudiness, crystallization, and bubbles must be avoided in the process of melting, cooling, and subsequent re-heating. Fourth, it must be possible to bring the glass from the plastic to the solid state without producing stress. Fifth, glass must not be tarnishable or hygroscopic. Sixth, it must be colorless and physically strong enough to bear the manipulation necessary in grinding and polishing.

Beside silicic acid, the only glass-making acids were boric acid and phosphoric acid and perhaps arsenic acid. There was a tradition that these acids only gave tarnishable glass, but experiments showed that phosphoric and boric acids could be combined with many metallic oxides and in addition to the six usual elements, namely, silicon, potassium, sodium, lead, calcium, and oxygen, the following were introduced by degrees in quantities of at least 10 per cent: boron, phosphorus, lithium, magnesium, zinc, cadmium, barium, strontium, aluminium, beryllium, iron, manganese, cerium, didymium, erbium, silver, mercury, thallium, bismuth, antimony, arsenic, molybdenum, niobium, tungsten, tin, titanium, uranium, and fluorine.

It was soon seen that by the introduction of new elements the variation of the hitherto fixed relation between refraction and dispersion could be attained. On the other hand, very few of the elements rendered the dispersions of crown and flint