| Summary: | Fifty years ago very little was known about conditions in the earth's atmosphere above a height of a few kms. and such ideas as did exist were mainly speculative. Since then, however, much work has been done, powerful new techniques developed, and a store of information accumulated about the inaccessible region above the maximum height of balloon ascent where no direct measurement has been possible until the recent development of high altitude rockets. Indirect methods have been used and one of the most successful of these has been the spectrographic study of the light emitted by these regions. The most intense source of this is the "northern lights" or aurora polaris. Although this phenomenon has been observed and classified for many years - Seneca, in the first century A.D. gives a description of it.- it was not until iTegard and his collaborators in the second decade of the present century analysed its light spectrographically that it was used to provide qualitative information about the condition of the upper atmosphere. The aurora occurs irregularly and is most frequent in a region known as the Auroral zone centred 23.5° from the poles. In addition to this transient phenomenon .Rayleigh found that there was emitted from the earth's lines of the spectrum. This was not a very satisfactory state of affairs as it would have been of great interest to have examined and compared spectra obtained from different phases of a display. However, it was found that if the size of these prism instruments was increased, very little gain in sensitivity was achieved as the absorption of the large prisms involved and the reflections from the many optical surfaces required effectively reduced the theoretical gain to negligible proportions. Thus the desire to have simultaneously high resolution and high light power was found impossible with prism instruments using traditional optics. Vegard4overcame this in his later work by using low dispersion instruments of maximum power to investigate the variations in intensity of the more ...
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