Two similar thin equi-convex lenses, of focal length f each, are kept co-axially 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 ( $\mathrm{\mu }$= 1 .5) as that of glass) then the equivalent focal length is F_2. The ratio of F₁ & F₂ will be