absorption continues slightly on into the violet, gradually fading away until the transmission becomes nearly complete for the bright blue mercury line (435 μμ). In examining b, c and d of Plate 1 it must be remembered that the second order ultra violet overlaps the first order so that the group near 365 μμ appears in the first order at the extreme right of the figure and in the second order at the extreme left. In d of this Plate the arc spectrum fades off on the left, not from absorption but from the weakening of the photographic action. The Euphos glass is extremely transparent to the radiations throughout all except the extreme violet of the visible spectrum, and well into the infra red, as will hereafter be seen. The results here obtained for its absorption of the ultra violet are altogether parallel with those shown in the paper by Schanz and Stockhausen and also by Hallauer. The Euphos glass thus enables a particularly clean partition of the visible spectrum from the ultra violet and extreme violet to be made. If it were possible to obtain an equally good absorbent for separating the infra red from the visible spectrum radiometric measurements of efficiency would be greatly facilitated. It should here be noted that Euphos glass appears in various shades and some imitations of it are now upon the market, so that a sample of such glass should be tested in the spectrograph before use for such a purpose as the present, inasmuch as in some of the shades the cut-off of the ultra violet is much less sharp and complete. The sample here used was the original No. 1, 2 mm. thick. Method of Investigation. The method taken for the evaluation was the familiar one of measuring the radiation directly by means of a thermopile connected with a sensitive galvanometer in a manner familiar in recent experiments on the efficiency of illuminants in the visible spectrum, e. g., Lux, Féry." The thermopile was chosen as the radiometric instrument merely as a matter of convenience. The instrument actually used was a Rubens linear thermopile, having 20 constantin-iron couples with a total resistance of 4.6 ohms. It was mounted as shown in Figure 1, in a vacuum tube with a quartz window immediately in front of the couples. The inner body of the instrument, containing the couples, was taken out of its original mounting and set up in a tube about 37 mm. in diameter, through the upper end of which was sealed a pair of leading-in wires. Zts. f. Augenheilk., May 1910, Table VII, figure 3. These were firmly clamped in the binding posts of the instrument by working through the side tube attached for the reception of the quartz window. The thermopile was then pushed up exactly opposite the side tube and wedged in place with cork and cotton wool attached with shellac. The end of the side tube was flanged out and ground flat for the fitting of the quartz window and after the shellac had dried out thoroughly the window was fastened in place and the lower end of the tube drawn out for the attachment of the pump. The tube was pumped to the high vacuum usual in an X-ray tube, and was then sealed. It was mounted as shown in a block of wood to which was secured the disconnecting terminal, reached by a long handled plug, FIGURE 1. Vacuum thermopile. FIGURE 2. Quartz cell. and the whole was then surrounded by a pasteboard case having a hole just opposite the quartz window, and packed full with loose cotton wool. The galvanometer was of the D'Arsonval type, having a sensibility of 2×10 ampere per mm. scale deflection. Its period for the attainment of a complete deflection, was, under the ordinary conditions of its use, 1 minute. The galvanometer deflections were read by a scale and telescope, the scale being a special one bent to 1.5 meters radius. The thermopile indications were calibrated in absolute measure by observations on the radiation of a standard incandescent lamp supplied by the Bureau of Standards. After applying the proper correction for stray thermal losses and spherical reduction factor and reducing the readings as taken to the standard distance of 50 cm. employed throughout this investigation, the constant of the thermopile galvanometer system was found to be 1 mm. = 1 scale division = 35.3 ergs per second per square cm. By this constant the observed deviations were reduced to absolute dynamical measure. As a matter of convenience and to establish an approximate ratio between the ultra violet radiation from the various sources studied and the radiation in the visible spectrum, an absorption cell which eliminated nearly all the infra red was kept in front of the thermopile window. This cell, Figure 2, was of glass, ground flat and exactly 1 cm. thick, 44 mm. external diameter and 35 mm. internal diameter. The glass ring was provided with a hole for filling and was closed by two quartz plates cut across the axis, each 2.25 mm. thick and 44 mm. diameter. These were fastened with hard shellac to the glass cell, and the cell in use was filled with distilled water. The absorption of a layer of distilled water of this thickness is shown in Figure 3 taken from Nichols's experiments.10 Quartz has no material absorption in the part of the infra red spectrum transmitted and neither quartz nor 10 Nichols, Physical Review, Vol. 1, p. 1. distilled water in this thickness has any material absorption in even the extreme ultra violet up to the limit investigated. The use of this cell therefore could produce no sensible effect on the accuracy of the ultra violet measurements, while it did serve the extremely useful purpose of limiting the total amount of energy to be measured and of eliminating any difficulties that might arise owing to absorption in the further part of the infra red, all the absorbing media incidentally used being, as compared with water, practically entirely transparent to all the radiations that got through the water cell. It would have been convenient if some substance cutting off the infra red sharply at 750 μμ or 800 μμ had been available. Unfortunately, there is no such substance, so far as has yet been discovered, the very few substances less transparent than water in the region 800 to 1300 μμ being useless for the purpose of this investigation on account of opacity in the ultra violet and generally in the visible spectrum as well. Iron ammonium alum used by Lux (loc. cit.) and the copper salts used by Féry (loc. cit.) are open to this objection and the same is true of all the otherwise useful and promising substances discussed in the very thorough and valuable researches of Coblentz.11 In some of the experiments a second similar quartz cell was used, particularly in work on arc lamps. In this case the Euphos glass used to cut out the ultra violet portion of the spectrum was permanently affixed to one of these cells and either the plain quartz cell or the Euphos-quartz cell was thrust into the beam so as quickly to get differential readings. In order to avoid the somewhat large correction due to reflection of energy which would have been produced by the introduction of a plain slip of Euphos glass to cut out the ultra violet the following expedient was adopted. The Euphos glass was attached to the surface of the quartz cell by spring clips with the addition of a thin capillary film of pure glycerine between the quartz and glass surface. Glycerine is immensely transparent to all radiations, including the extreme ultra violet, to which Canada balsam and gelatine are quite opaque. Its index of refraction, 1.47 for D, is sufficiently near that for quartz and the various glasses to reduce the loss of light at the surfaces to an entirely negligible amount. As the Euphos has a slightly less index of refraction than quartz, there was a minute residual gain in the total transmission of the system when the Euphos glass was added, in the right direction to compensate for the minute losses by absorption in the glycerine film. 11 Bull. Bureau of Standards, Vol. 2, p. 619. As a check on the possible magnitude of this virtual absorption by the glycerine film readings were taken on a tungsten lamp through the quartz cell alone, and through the quartz cell plus a disc of optical crown glass 2 mm. thick secured with glycerine in the ordinary manner. The absorption of this crown glass is shown in Plate 1, e, f, g, in which e is the spectrogram of the quartz arc taken with a wide slit and 2 minutes exposure, f the spectrogram through the crown glass in question, and g through the Euphos glass. In spite of the fact that there is a slight absorption by the crown glass in the region near 300 μμ, the addition of the crown glass and glycerine film reduced the galvanometer deflection by barely 0.5 %, an amount scarcely outside the errors of observation. The energy cut off from the spectrum of a tungsten lamp by the crown glass would be of course very small, but perhaps not negligible, since as Schanz and Stockhausen have shown (loc. cit. table VIII, figure 6) the tungsten lamp spectrum goes quite down to 300 μμ in sufficient strength to give a clear photographic effect. At all events it is evident that the use of the glycerine film involves no material errors. In the ordinary experimentation in using steady sources, sets of readings were taken alternately with and without the Euphos glass, the glass being either added to the clear cell with the glycerine film, or removed and the film quickly washed away with distilled water. With sources which give trouble from unsteadiness the second quartz cell was brought into play as previously mentioned. Aside from a slight drifting of the zero point, which is generally observable in measurements with a thermopile, the method adopted worked very smoothly. The drift, however, was usually small and slow and satisfactorily taken care of by a time correction. With proper attention to this, the readings, although necessarily slow, were nearly as consistent as would be found in ordinary photometric measurements. The following string of deflections forming a single group of 5 readings is typical of those obtained under ordinary conditions. Scale readings from bare quartz lamp through quartz cell only. |