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 F₁ . When the space between the two lenses is filled with glycerine (which has the same refractive index (μ =1.5) as that of glass) then the equivalent focal length is F₂ . The ratio F₁ : F ₂ will be