Coronal index of solar activity. IV

The values are reported of the coronal index of solar activity for the years 1964 to 1970, determined using the method described by Rybansky. The index is expressed in terms of the total energy emitted by the whole visible corona in emission line Fe XIV - 530.3 nm. A graph showing the behaviour of the index during the 20th cycle of solar activity (monthly averages) is given. A conspicuous decrease in activity at the end of 1970 can be observed. At this time, the photometry method at the Lomnicky Stit Observatory was changed and the results were blamed on this change of method. Several tests were made and it was proved that the observed change in activity was real