Jai Prakas are twin hemispherical bowl instruments, each a reflection of the sky above, and marked in sectors and gap regions.
The bowls are complementary, in the sense that, the gap region in one bowl is the sector region in the other and vice versa. The idea being that, the observer needs to be inside the bowl, to take readings - which means that readings would not be possible in the regions where the observer would be able to walk - and hence the complimentary bowl.
Cross wires are stretched in the North-South and East-West direction on the surface of the instrument bowls. Shadow of the centre of this cross wire, on the surface of the bowl, shows the position of the Sun in the sky. The sectors on the surface of the hemisphere are marked with altitude and azimuth circles, diurnal circles, the tropics and intermediate circles and markings to determine the approach of a sign of the Zodiac, on the Meridian.