We have used the Very Long Baseline Array ( VLBA ) and the Very Large Array ( VLA ) to image Galactic neutral hydrogen in absorption towards four compact extragalactic radio sources with 10 milliarcsecond resolution .
Previous VLBA data by Faison et al ( 1998 ) have shown the existence of prominent structures in the direction of the extragalactic source 3C 138 with scale sizes of 10-20 AU with changes in HI optical depth in excess of 0.8 \pm 0.1 .
In this paper we confirm the small scale H i optical depth variations toward 3C 147 suggested earlier at a level up to 20 % \pm 5 % .
The sources 3C 119 , 2352+495 and 0831+557 show no significant change in H i optical depth across the sources with one sigma limits of 30 % , 50 % , and 100 % .
Of the seven sources recently investigated with the VLBA and VLA , only 3C 138 and 3C 147 show statistically significant variations in HI opacities .

Deshpande ( 2000 ) have attempted to explain the observed small-scale structure as an extension of the observed power spectrum of structure on parsec size scales .
The predictions of Deshpande ( 2000 ) are consistent with the VLBA H i data observed in the directions of a number of sources , including 3C 147 , but are not consistent with our previous observations of the H i opacity structure toward 3C 138 .