Why does the images of slit slip down in bottom as we move the convex lens in the experiment of "determination of wavelength of sodium light by fresnel biprism experiment". We made the system coaxial and then introduced the convex lens to find the slit width by adjusting the lens in two positions where virtual images of the slots are clearly seen. But one time images appear above the cross wire. Another time in another position of lens it appears below the cross wire. Is it normal or we should conclude our lens isn't coaxial to the setup?