Abstract: A highly sensitive measuring cell for a differential refractometer of the interference type for chemical analyzers comprises two channels in a measuring body, one of said channels conveying a medium whose refractive index is to be measured and the other channel conveying a reference medium of known refractive index. The respective path lengths of the measuring channel and the reference channel are mechanically adjustable to exactly the same lengths in order that a maximum common mode rejection ratio (hereafter "CMRR") value may be achieved.