Two slits `S_(1)` and `S_(2)` illuminated by a white light source give a white central maxima. A transparent sheet of refractive index 1.25 and thickness `t_(1)` is placed in front of `S_(1)`. Another transparent sheet of refractive index 1.50 and thickness `t_(2)` is placed in front of `S_(2)`. If central maxima is not effected, then ratio of the thickness of the two sheets will be :