Why can we see virtual images? It cannot be formed on the screen. The retina in our eye acts as a screen. If the image cannot be formed on the screen, why can we see it?

It is better to have brief account to working mechanism of eye before reaching out this question. 

Whenever the rays of light coming from virtual object i.e. divergent rays of light fall in our eye lens which is a convex lens, it converges the rays of light forming the real image in our retina making us able to see the objects.