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 glycerin (which has the same refractive index ($\mu=1.5$) as that of glass) then the equivalent focal length is $F_2$. The ratio $F_1:F_2$ will be: