An optical device is constructed by fixing three identical convex lenses of focal lengths $10 \mathrm{~cm}$ each inside a hollow tube at equal spacing of $30 \mathrm{~cm}$ each. One end of the device is placed $10 \mathrm{~cm}$ away from a point source. How much does the image shift when the device is moved away from the source by another $10 \mathrm{~cm}$ ?