Fringes are obtained with the help of a biprism in the focal plane of an eyepiece distance $1\;m$ from the slit. A convex lens produces images of the slit in two positions between biprism and eyepiece. The distances between two images of the slit in two positions are $4.05 \times {10^{ - 3}}\;m$ and $2.90 \times {10^{ - 3}}\;m$ respectively. The distance between the slits will be