Thomas Young used a single slit between the light source and the double slits. I can't understand why did he used the single slit, since the light from only one source is coherent already or isn't it? Does the narrow single slit make incoherent source coherent?
At the time of Young there were no lasers to provide coherent light. Incandescent light is incoherent because it comes from many sources and the same is true for sunlight. By passing the light through the one slit
he created a single coherent wave front . It is worth reading his description "on the nature of light and colors" in the link above.
Edit: I have not addressed the "why" a slit induces coherence. It has to do with wavelength : the size of the slit must be such that, similar to a diffraction grating, it diffracts specific wavelengths differently and also acts as a point source creating a coherent wavefrong. Here is an illustrated description of coherence. Here is a blog entry discussing coherence from thermodynamic sources .