The normal pattern that you see is shown in the diagram below.
Each slit produces a diffraction pattern (left hand diagram) which overlap and are seen a one diffraction envelope (red dotted line in right hand diagram) because of the slit separation being small produces.
This diffraction envelope modulates the intensity of the two slit interference pattern (red full line in right hand diagram.
The width of the single slit diffraction pattern is controlled by the slit width and the separation of the interference fringes is controlled by the separation of the slits.
If the slit separation is increased then the separation of the interference fringes decreases.
If the slit separation is large compared with the slit width then the diffraction patterns from the two slits can be resolved as shown in the diagram below.
Interference fringes will now be seen where the light from the two slits overlap as shown in the diagram below.
The separation of the fringes is now much smaller as is their intensity.
If the separation of the slits is further increased there will come a time when it will not be possible to observe the interference fringes.
The Rayleigh refractometer in its simple form is a device which uses the interference from two slits with a relatively large separation.