PENETRATION OF RADIATION THROUGH FOG AND HAZE
It is now of interest to survey some of the actual observations which have been made on the penetration of radiation through fogs and haze and to consider the degree of success which can be expected in increasing the range of vision by photography with radiation of longer wavelength.
In order to see whether some wavelengths are transmitted better than others, experimenters have studied natural fogs out of doors, as well as artificial fogs produced in the laboratory. There is much variation in the significance of the results obtained, partly owing to the absence of a knowledge in many cases of the sizes and variation in sizes of the fog particles; partly owing to the way the measurements were made, some observers merely recording the "visibility" or "perceptibility" of distant objects, some using visual and others physical methods of measuring the intensity of light transmitted, some taking photographs of distant objects, some using relatively broad spectral bands obtained by the use of filters, and others narrow bands produced in the spectrograph.
We shall consider first of all some observations that have been made in the laboratory using artificial fogs, then physical or visual methods on natural fogs, and finally actual experiments made by photography through fog and haze.
Observations on a number of artificial fogs produced by condensing steam on particles of dust or other nuclei present in the air have shown in some cases a definite maximum transmission of light at a particular wavelength. Utterback111 found such a