Jets through a non-circular orifice has been studied by Niels Bohr (1909) in his early career. He used this method to determine the surface tension of water by measuring the axial wavelength of these jets. He also proposed non-linear corrections which are validated by experiments done by Moon et. al. (2016). Recent experiments on steady-state capillary jets by Ganan-Calvo et.al. (2019) shows the regime where body forces are not important. In this light, we perform a normal mode analysis with viscosities of both fluids i.e. of jet and outside the jet to predict the axial wavelengths. We can then predict the wave formations of both liquid jets and gas jets with viscous effects. We verify our results by direct numerical simulations using an open-source fluid solver Gerris (Popinet, 2009).