The paper discusses an investigation of linear polarization produced by the transmission of light through an astronomical spectrograph slit. An experimental apparatus was designed and set up to carry out this work. The parallel beam of plane polarized light was rotated in the measuring system, by optically active of half-wave plate. The intensity of normally incident of polarized light of wavelength 0.436 μm transmitted by various slit was measured as a function of slit width. The results indicate:(1) That the degree of polarization increased as thedepth of the narrow slit was increased. (2) That the degree of polarization increased as thewidth of the narrow slit was decreased. (3) That when the width of slit was widened the degree of polarization tended to approach a constant value asymptotically. (4) That the theoretical calculation of Slater (1942) predicts the measured experimental values more accurately than Thiessen (1947) and Jones and Richards (1954). It is shown that the existence and order of magnitude of all these effects may be predicted from the propagation losses of the electromagnetic theory of light in metallic wave guide. The paper brings out the salient points related to the degree of polarization of light by a dielectric slit of finite depth. The polarization effects from one such slit have been investigated and the results were compared with those for metallic slits.