The invention concerns a method for the optical analysis of a fluid, in which the fluid that has a first refractive index, is surrounded by an auxiliary fluid with a second refractive index, the first refractive index being larger than the second refractive index and light being sent through the fluid. It is endeavoured to obtain an encapsulation of the fluid in the auxiliary fluid at low cost. For this purpose, firstly the auxiliary fluid is led into a measuring channel, until it is filled, and then lead the fluid into the auxiliary fluid within the cross section with a flow speed, which generates a parabolic flow profile.