A thin equi-convex lens of flint glass, refractive index $$\mu_1$$, is kept coaxially in contact with another thin equi-concave lens of crown glass, refractive index $$\mu_2$$. The system is completely immersed in water of refractive index $$\mu_w = \frac{4}{3}$$. Parallel rays of light incident parallel to the principal axis in water are focused by this system at a distance of $$24\,\text{cm}$$ beyond the system. The thickness of the system is negligible. If the radius of curvature of each surface is $$R = 20\,\text{cm}$$, the difference $$\left(\mu_1 - \mu_2\right)$$ is