From the data on Cherenkov light from EASs collected over the period of more than 15 years , the spectrum of cosmic rays was obtained in the energy range 10 ^ { 15 } -10 ^ { 18 } eV .
This spectrum has two features : at \sim 3 \times 10 ^ { 15 } eV ( first knee ) and at \sim 10 ^ { 17 } eV ( second knee ) .
The first knee is characterized by the indexes \gamma _ { 1 } = 2.7 \pm 0.03 and \gamma _ { 2 } = 3.12 \pm 0.03 and the second knee — by \gamma _ { 1 } = 2.92 \pm 0.03 and \gamma _ { 2 } = 3.24 \pm 0.04 .
In the second case the difference amounts to \Delta \gamma _ { 23 } = 0.32 \pm 0.03 \pm 0.05 which is less than the case of the first knee \Delta \gamma _ { 12 } = 0.42 \pm 0.03 \pm 0.05 ( here dual errors represent statistics and systematics correspondingly ) .
The lesser difference \Delta \gamma _ { 23 } can be explained with the influx of cosmic rays from Meta-galaxy and , hence , with some increase of the cosmic rays intensity in the energy range 5 \times 10 ^ { 16 } -10 ^ { 18 } eV , which compensates the escape of heavier nuclei from the Galaxy .
The presence of the second knee could be confirmed by abrupt change in mass composition from from \left < \ln { A } \right > \sim 3 at \sim 10 ^ { 17 } eV to \left < \ln { A } \right > \sim 1.5 at 10 ^ { 18 } eV found from the analysis of longitudinal development of EAS .