Experimental results of atmospheric light transmission tests for three years, from March 1970 to March 1973, are described. The experiments were performed in the downtown area of Tokyo. A He-Ne laser at the wavelength of 0.63 µm and a light emitting diode at the wavelength of 0.85 µm were used as light sources, which were modulated by PCM signals at a bit rate of 122 Mb/s and 6.3 Mb/s, respectively. It is confirmed that the cumulative probability distribution of lightwave attenuation can be estimated precisely by that of the visual range. Based on the estimation, it will be necessary to use a tandem optical transmission system through the atmosphere with repeaters at intervals shorter than 1 km, in order to attain a higher reliability of about 99.9 percent. In addition, this paper clarifies the dependency of optical pulse atmospheric transmission upon weather conditions under the condition of constant received light power.