When a ray of light passes from a denser medium (such as glass or cedarwood oil) into a rarer medium (such as air), it typically bends away from the normal, a phenomenon known as refraction.
However, when cedarwood oil is placed between the microscope slide and the objective lens,
light passes through more directly with minimal scattering.
This is because the refractive index of cedarwood oil is close to that of glass.
Normally, when light moves from a denser medium (like glass) to a rarer medium (like air), some light gets scattered, reducing image clarity. By placing cedarwood oil, which has a similar refractive index, the scattering of light is minimized, and more light enters the objective lens directly.