Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is $F_{1}$.. When the space between the two lenses is filled with glycerine (which has the same refractive index $\left ( \mu =1.5 \right )$ as that of glass) then the equivalent focal length is $F_{2}$. The ratio $F_{1}$ : $F_{2}$ will be: