It will be noted that the smallest percentage of ultra violet is shown again by the G. E. M. lamp, with the acetylene flame standing second. The Welsbach mantle of the Graetzin lamp runs materially higher than any of the electric incandescent lamps in spite of the fact that this lamp was tested with its globe on. Next higher than the Graetzin lamp, and approximating the arc lamps, comes sunlight, standing between the incandescent sources which give a continuous spectrum and the arcs of various sorts which give highly selective radiation. At the other end of the list is the quartz lamp worked intensively without its globe. These ratings of the various illuminants are instructive as showing the distribution of the energy as between ultra violet and the remainder of the spectrum, but they are not significant as regards the extremely practical matter of illumination. If the ultra violet component of artificial light involves any risk of injury to the eyes the one important thing to find out in comparing sources of light is how much ultra violet they deliver for a given illumination. In other words if one desires to light a room, say to an intensity of five foot candles, with what illuminant can he obtain this intensity while receiving the minimum amount of ultra violet radiation? It is not of the slightest practical consequence from the standpoint of good and safe illumination whether a given light source produces much or little ultra violet per watt, provided it produces an insignificant amount per foot candle, hence the luminous efficiency

of the source is in the last resort the thing which determines the presence or absence of ultra violet radiation in material amount. In other words the more efficiently the energy supplied to the illuminant is transformed into light the less important does the ultra violet radiation become in considering the source as a practical illuminant.

Table III assembles the commercial light sources tested, with respect to the ultra violet energy accompanying a given illumination. The first column of the table gives merely for the purpose of record the actual deflections found to be due to the ultra violet energy, and column two the total ultra violet radiation in ergs per second per square cm. per foot-candle of illumination. At the head of the list stands the quartz mercury arc with its diffusing globe. Of the commercial illuminants tested this gives by all odds the smallest proportion of ultra violet per foot candle. As the previous tables show, the ultra violet energy of this source so equipped is small from any point of view. Its unique position, however, is due largely to the fact that the light-giving radiation, which lies practically at the very peak of the luminosity curve for vision, is produced at enormous efficiency, according to Buisson and Fabry 17 not less than 55 candles per watt for the green line at wave length 546 which supplies nearly two thirds

17 Comptes Rendus, Vol. 153, p. 254.

of the total light and at almost as high efficiency for the pair of yellow lines which supply nearly all the rest. Next in the list, a rather bad second, comes the Graetzin gas lamp, its position again being due to the somewhat selective radiation that gives it a very high luminous efficiency. Third, comes the G. E. M. lamp which, from its relatively low temperature, gives a small absolute amount of ultra violet radiation, although its luminous efficiency is not great.

At the other end of the line comes the special enclosed arc with 91 ergs per second per square cm. per foot candle, and next to it the quartz lamp without its globe. Of course the quartz lamp without its globe is never used for illuminating purposes, but only for such work as sterilization of water and the like in which the ultra violet rays are the things sought. Operated for this purpose it undoubtedly is the most efficient powerful source of extreme ultra violet. To test this feature of the matter energy measurements were taken on the two quartz lamps without their globes and on the magnetite lamp free from its globe while using as a screen instead of the Euphos glass a disc of the very light crown glass previously referred to, which practically effects a separation at wave length 300 μμ absorbing substantially all the energy below this point and transmitting at almost full intensity the rest. The result of this test, measuring the extreme ultra violet and reducing it to the mean spherical output of ultra violet, showed for the extreme ultra violet efficiency of the new quartz lamp 4.07% and for the efficiency of the old quartz lamp 3.14 %. A similar measurement of the magnetite arc showed an extreme ultra violet efficiency of 1.13%. These figures may be properly compared with the tests for the ultra violet efficiency of the quartz lamps made by Fabry and Buisson. 18 In this case two mercury lamps showed respectively extreme ultra violet efficiencies of 6.4 and 4.7%, the ultra violet separation being effected by the screens used by Fabry and Buisson at wave length 320 μμ. The values obtained by the French investigators and in this study therefore check each other closely, showing that in the quartz mercury lamp 4 to 5% of the total input is returned in the form of extreme ultra violet radiation when the lamps are operated, as they are for sterilization purposes, without their globes. The lighting power of the lamp falls off very greatly in this condition.

When operated with the globe the total proportion of ultra violet becomes both absolutely small and extremely small relatively to the

18 Comptes Rendus, Vol. 153, p. 93.

light given. In this connection the position of sunlight in Table III is not without importance. On the face of the returns it has a less amount of ultra violet with respect to the illumination than most of the artificial illuminants. This is due to the very high temperature of the source, which insures high luminous efficiency, in connection with strong ultra violet absorption in the atmosphere. Unfortunately one can apply Planck's law to very few practical sources of light. The sun is ruled out by the very erratic and highly selective absorption which produces the Frauenhofer lines and also by an unknown absorption of the extreme ultra violet which may take place in the earth's atmosphere or near the solar surface or in both places. Incandescent lamps involve absorption by their globes and also in the case of more recent ones a certain amount of selective radiation. The whole tribe of arcs which yield in a greater or less degree discontinuous spectra, for which Planck's law does not hold, are also thereby eliminated, so that this otherwise very useful guide to the distribution of radiation ceases to have exact significance.

The ultra violet component of sunlight has been considerably disputed. It has been held by some investigators like Dr. Voege 19 that sunlight contains more ultra violet than the arc light, while Schanz and Stockhausen 20 take the opposite view. In a sense both are right and both wrong. Sun

50

light undoubtedly contains only a very modest proportion of ultra violet per foot candle of illumination when one considers direct sunlight alone. If, however, one considers the total daylight effect, including skylight under favorable circumstances, the situation takes on a totally different aspect. The light diffused by the blue sky is mainly violet and ultra violet, being substantially that light of which the direct sunbeam is robbed by scattering. Figure 6 shows in curve 19 The Illuminating Engineer, Lond., Vol. II, p. 205. 20 The Illuminating Engineer, Lond., Vol. I, p. 1049.

A the distribution of energy in the directly received solar light. Curve B shows the distribution of energy in the diffused light of the blue sky when the total of this diffused energy equals 20% of the total directly received solar energy, a not uncommon proportion. It will be noted that the maximum for this curve is in the far violet near the edge of the ultra violet. Curve C is the summation of A and B and it will be seen at once that the proportion of ultra violet is something like three times as great as in the case of the direct solar rays. This proportion would raise the ultra violet activity of daylight to a point higher per foot candle than that reached by any ordinary artificial illuminant.

Weisner 21 in photographic observations of light received on horizontal surfaces states for example, "For solar altitudes less than 19 degrees the chemical intensity of the sunlight as compared with diffused daylight is negligible, with increasing solar altitude it gains in comparison with the diffused daylight. *** Since the intensity of the direct beam may reach twice that of the diffused, the total combined chemical effect may be three-fold that of the diffused light."

Daylight, therefore, varies very greatly in ultra violet energy, ranging from the low value given in Table III for direct sunlight to values which would exceed almost all artificial light sources. The chief claim of sunlight to serious consideration from the standpoint of ultra violet energy, however, lies in the very large amount of energy which the sun delivers. There is considerable doubt as to the exact amount of solar radiation outside of the atmosphere, but that which gets through the atmosphere is pretty well determined and its amount, from the data given by Abbott 22 amounts practically, under favorable conditions, to not less than 1 kw. per square meter, which is 0.1 watt per square cm. If one assumes that only 10% of this is in the ultra violet region, an amount which may be exceeded at times, the total ultra violet radiation rises to 105 ergs per second per square cm., several times that given by the most powerful artificial sources of ultra violet at even a distance of so short as half a meter.

Considering this very large flux of ultra violet energy it is small wonder that troubles from sunburn and snow-blindness are not infrequent. Did we not habitually shield our eyes by interposing the rim of the hat or the brow and by systematically looking away from the direct sunlight ocular troubles would be common and severe.

21 Denkschriften Wien. Akad., Vol. 64, 1897.

22 "The Sun", Chapter VII.