An of-axis Fresnel zone plate, through use of a halftone screen to introduce a spatial carrier frequency, can be used as a large-area, coded aperture for imaging incoherent radiation sources. This paper discusses the structure of off-axis Fresnel zone plate as an aperture and the method of imaging from the viewpoint of the spatial resolution by optical simulation, appending some examples of objects, holograms and reconstructed images. It is then concluded that the of axis Fresnel zone plate having D (diameter) =5.8 cm and νe (spatial center frequency) =23 l/cm is sufficient for the reconstructed images.
Zirconium in natural water samples was determined spectrophotometrically after concentration using95Zr as a yield tracer. Acidified sample (1 l) with a known amount of95Zr was evaporated to dryness, treated with hydrofluoric and nitric acids, and then with nitric and perchloric acids, and evaporated nearly to dryness. The residue was taken up with 2 M perchloric acid and zirconium in the solution was extracted with 0.1 M thenoyltrifluoroacetone (TT-A) in benzene. The TTA complex solution was converted to hydrochloric acid solution. After the recovery yield was estimated by measuring the radioactivity of95Zr in the final solution, zirconium was then determined by spectrophotometry with Arsenazo III. Zirconium contents in lake and hot and cold spring samples from Nikko and Shiobara districts were found to be 0.29-2.8μg/l by this method.
Since Japan belongs to the temperate monsoon climate, thee seasonal difference of the origin of air mass is very clear. Thus the tritium concentration in precipitationn changes seasonally ccording to the origin of air masses. The author measured the tritium concentration in precipitation of Tokyo for every ten-day period from August 1972 to May 1974. Judging from the daily synoptic weather chart, the rainfall-inducing air masses in Japan were classified into five types; polar maritime air mass (Pm), polar continental air mass (Pc), tropical maritime air mass (Tm), tropical continental air mass (Tc), and equatorial maritime air mass (Em) (Fig. 3) . And the precipitation for every ten-day period sampled for tritium measurement were classified into these five types (Table 2) . Based on this classification, it is confirmed that there exist clear difference in the tritium concentration between the rainfall from the continental air mass and ones from the maritime air mass (Table 3) . It is characteristic that the tritium concentration in rainfall induced by.equatonal maritime air mass such as typhoon in summer and early fall season is very low whereas the tritium concentration in rainfall and snowfall induced directly by the polar continental air mass in late winter season is very high. The regional difference of the tritium concentration in intermonthly precipitation could considerably be explained by this synoptic meteological classification of rainfall-inducing air mass. In spite of these regional differences rences of tritium concentration in precipitation, use of the tritium concentration of Tokyo as a representative value of Japan may be allowed because of the similarities of the changing pattern and annual mean tritium concentration. The time series variations of tritium concentration in precipitation of Tokyo from August 1972 to December 1977, Tsukuba from December 1976 to Apri1 1978, and Nagaoka from April 1977 to March 1978 are listed in Fig. 2.
In order to investigate the detailed absorption and excretion of 2, 4-DNT in the rat, after the single oral administration of 3H-2, 4-DNT, the time-course of radioactivity level in blood, liver and digestive organ contents, and of biliary, faecal and urinary excretion were observed. Radioactivity level in blood reaches a peak at 6 hr after the administration and its halflife was about 22 hr. Maximum level of radioactivity in the liver was observed at 6 hr after the administration as well as in the blood. In contrast with the radioactivity levels in blood and liver, amount of radioactivity in small intestine content is markedly decreased for 6 hr after administration. In the biliary excretion, about 10 per cent of the radioactivity administered was excreted in bile within 24 hr. The excretion rate is rising steadily from 6 hr after administration and its peak time was 9-10 hr. In the faecal excretion, the most of the excretion radioactivity was concentrated on 6-9 hr faeces as reflected with the biliary excretion pattern. On the other hand, the most of the urinary excretion radioactivity was concentrated on 0-6 hr urine.