Technical details are given for evaluating ultraviolet solar radiation by two closely agreeing methods: (a) by means of a balanced thermopile and filter radiometer, calibrated against a standard of thermal radiation, and (b) by means of a photoelectric ultraviolet meter calibrated against a standard of ultraviolet radiation. A photoelectric cell and filter method for determining the solar ultraviolet spectral-energy distribution is described. Data are given on ultraviolet solar radiation intensities observed at

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Ultraviolet-filter transmission measurements are made, using a newly devised p ortable precision ultraviolet meter (Research Paper RP647) and 4 glass filters, giving the spedal quality and total intensity in the band of ultraviolet solar radiation extending from about 2900 to 3500 A. At the high-altitude station (Flagstaff, Ariz.) data were obtained on solar ult.raviolet intensities under various conditions; high and low humidity, large and small air m asses, clear and sm oky skies. The data obtained at sea-level st ations (Washington, D. C., and San Juan, P. R.) a re of interest in connection with the question of ultraviolet intensities in the tropics as comp ared with similar stations at higher latitudes.Data are given on the amount of ultraviolet transmitted through light fog, and on the a m ount r eflected by the sky and by snow; also the effect of altitude and latitude on the sola r ultraviolet intensity at the earth's surface.The observations show that, while a light fog greatly reduces t he intensity (when the sun appeared as a distinctly outline d white disk, the intensity was reduced to one-tenth or less) the spectral quality was practically unchanged, owing to the relatively nonselective character of the transmission through water vapor. Similarly, the ultra violet refl ect ed from snow is extraordinarily high, owing to the fact that the ice crystals, within which the reflection occurs, are highly transparent to ultraviolet radiat ion.The absorption of ultraviolet is caused by atmospheric polution n ear t he earth's surface, and by ozone in the stratosphere . An increase of 40 to 50 percent in intensity in the band of wave length between 2900 and 3130 A, is observed in rising 2 km (7,000 ft) above sea level. A much greater increase in intensity may be expected at elevations of 15 to 25 km (10 t o 15 miles). However, such heights are not practicable for large-scale biological tests.The measurements at the Flagstaff station show a greater atmospheric transparency (less ozone) in the afternoon than in the forenoon ; and a greater transparency in the autumn than in the spring. This is in agreement with the work of Dobson and others, using other methods of observation.For the same solar altitude (air mass traversed by the rays) the ultraviolet solar intensities in the tropics (at San Juan) were somewhat higher than at a midlatitude, sea-level station (Washington) that is free from local air pollution . This is in agreement with other observations showing that, in the tropics, for the same solar altitude, the amount of atmospheric ozone is less, and consequently the ultraviolet intensities should be somewhat hi gher than in higher latitudes.However, this small difference in intensity, for the same solar altitude, does not appear to be sufficient to produce marked differences in biological effects.

A descript ion is given of a photoelectric ultraviolet-intensity meter and automatic integrating and recordin g apparatus for measuring the biologically effective component of ultraviolet radiation, of wavelengths 3132 A and shorter, from the sun and the entire sky, incident on a horizontal plane, under various meteorological conditions. Methods of standardization, in absolute value, are described (see RP1542 for supplementary data). A continuous graphical r ecord of the in tegrated daily total amount of biologically effective ultraviolet solar and sky radiation observed during a period of 3 years (1941 to 1943) in Washington, D. C. is given . The monthly totals of biologically effective ultraviolet, in absolute value (milliwatt minutes per square centimeter) mw min/cm2 are also given graphically. On the clearest days the biologically effective component of ultraviolet radiation of wavelengths 3132 A and shorter, incident from the sun and the whole sky at midday, ranges from about 180 microwatts per square centimeter, (p.w/cm 2 ) in midsummer to about 30 /Lw/cm 2 in midwinter. A series of erythema tests is given, correlating the physical (radiometric) measurements with the physiological reaction of the untanned skin, which information is of interest in helotherapy and bioclimatology.

Supplementing a previous report on plant pigment s (BS R esear ch Paper 617) the present paper d escribes new data on the infrared absorption spectra of plant and animal su bst ances (cellulose and proteilJs) as found in nature and, in some cases, in the form of pure m at erial prepared in t he laboratory.The dat a were obt ained by m eans of a mirror spectrometer, portable vacuum thermopile, a nd ironclad Thomson galvanometer, described in previous publications. Th, e r egion of t h e infrared spectrum extending to 15", was examined.Outst a nding among th~ plant substances examined are samples of pure rubber, and associat ed materials-styrene, indene, polystyrene, and polyindene.The cellulose mat erials examined included Cellophane (commercial prEiparation) , onion skin, translucent membrane of pith of pokeweed, the seed septum of inoonwort, and the seed wing of the cotton tree.The protejn m a t erials exami ned included dried specimens of chitin (the outer integument of i nsect s) , pit h of feathers, a ir bladder of fish, bat's wings, fil m of dried egg albumin, commercial gelatin, lining of egg shell, and wing of a d ragon fly.The miscella neous subst ances reported upon are polyvinyl acetate and pol yvinyl chloracetate resins, glyptal resin, shellac, and a group of substances of interest in connection with the Raman effect,