368 CURRENT "I~OPICS. [J. F. I.

to be slightly higher at the top and stirring vanes attached to a rod are provided for mixing the water prior to reading the temperature. Starting with a definite quantity of distilled water at 2o degrees C., the instrumen.t is kept in the circuit for a period of approximately two hours, provided the r.m.s, current does not exceed the rating of the resistance element or shunt. The test can, in fact, be started at any temperature which will permit it to be finished without bringing the water too near the boiling point. A correction factor is applied for initial temperatures which are more than 5 degrees above or below 20 degrees C. Equivalent current is read from temperature- time curves for different currents, determined from constant current readings. Results of careful laboratory tests show the error of the instrument is less than two per cent.

Some Special Applications of Pinhole Photography. ANON. (British Journal of Photography, vol. lxiv, No. 2959, p. 27, January I9, I917 . ) - -Very few photographers seem to appreciate what can be done with a pinhole when a lens suitable for a given purpose is not at hand. I t happens sometimes that no lens of sufficiently great or perhaps short enough focal length is available, or possibly one that will cover a sufficiently wide angle. If, however, we can put up wi,th less perfect definition and also long exposure, a great many things can be done with a simple pinhole, for focal length is a property that is practically infinitely elastic with a pinhole, while the angle of view can be any size up to very wide limits with a good hole. Also, depth is unlimited. As regards definition, the lack of sharpness is not nearly so great as is likely to be imagined by those unacquainted with what can be done. A good pinhole negative will give a lantern slide that will bear a considerable degree of enlargement on the lantern screen without any undue fuzziness. There is usually a certain amount of grain apparent, owing probably to the peculiar diffraction effects which go to form the image, but this need not be seriously objectionable. As regards focal length, we may consider anything feasible from about 40 inches down to an inch or less, consequently

a t small expense it is possible to secure effects that otherwise would be out of the question, neither 4o-inch nor 1-inch lenses being avail- able, as a rule.

Some of the most interesting results obtainable with the pinhole are those produced with exceedingly short extension, say an inch or so, for then we can get some idea of what the world looks like to some of the small creatures that inhabit it. Depth problems are imma- terial to the pinhole, as all distances are alike to it. Using a camera consisting merely of a metal lantern slide box with a pinhole fitted with a small shutter, most interesting results in under-water photog- raphy have been obtained. For more ordinary purposes, it is often desirable to see beforehand on the focussing screen what amount of the subject is to be included on the olate. It is not possible to see the pinhole image on the screen, but if we substitute for the pinhole a

March, I917.] CURRENT TOPICS. 369

hole about a quarter of an inch in diameter, we can see the subject through it from any part of the plate area. In this way we prac- tically turn the camera into a finder of the frame type, and this is all tl~at is necessary, actual focussing being neither required nor possible.

Ferro-cerium and Other Pyrophoric Alloys. C .R. ]~OHM. (Metal Record and Electropl~ter, vol. iii, No. I, p. 8, January, 1917. ) --About the year i9o 3, Welsbach discovered that certain alloys of the rare earths, when filed, gave off showers of bright sparks which would readily ignite inflammable gases. It was subsequently found that the best results were secured when employing an alloy carrying about 65 per cent. cerium earth metals and about 35 per cent. iron. The same action takes place with these alloys as wi.th the flint and steel, except that a spark is more easily produced than with the primi- tive tinder-box. The mixture of cerium earth metals, often called misch metal, consists mainly of the elements cerium, lanthanum, didymium, neodymium, praseodymium and samarium. All of these metals are white or light yellow in color and are not readily oxidized in air. When first produced, the price of ferro-cerium alloys was $60 per kilogram. Later, this was reduced to $12 per kilogram, and in 1913 to about $4.60 per kilogram: The present return to the old price is, of course, due to the abnormal conditions which have affected nearly all products.

While the original pyrophoric alloy or ferro-cerium manufac- tured by Wels.bach contains nearly 40 per cent. of iron, the compet- ing products possess only about 15 per cent. of iron, and for the pur- pose of hardening, about 2 per cent. of antimony or bismuth is added. Silicon is, of course, found in nearly all ferro-cerium alloys because it is either contained in the raw misch metal as an impurity, or has been absorbed from the clay crucibles in which the alloy is often pro- duced. T o insure a low-melting and smooth-casting pyrophoric alloy, certain manufacturers add about 5 per cent of copper. Most manufacturers produce ferro-eerium by the electrolysis of the an- hydrous chlorides of cerium, lanthanum, didymium, etc., and it is necessary that cheap current be available. Many manufacturers use ordinary graphite or clay crucibles with large iron cathodes; others use water-jacketed iron crucibles.

Welding Glass in Optical Contact. G.R. PARKER AND A. J. DALLADAY. (Enc~ncering, vol. ciii, No. 2663, p. 23, January 12, 19t7.)--For many optical researches glass cells are wanted, formed by two strictly parallel walls or windows, and different pieces of giass, lenses and prisms, etc., have frequently to be united in such a way as to keep the surfaces optically true. Glass surfaces can be polished without difficulty to be so nearly plane or so nearly of the same curvature that they show less than one interference fringe per inch when placed together and examined by light reflected from the