BRADY'S NEW NATIONAL PHOTOGRAPHIC GALLERY. – The new gallery which has been so long in preparation for Mr. Brady’s photographic establishment, at the corner of Broadway and Tenth street, will be opened this week. Report speaks of it as being unique of its kind, the space devoted to it and the elegance of its decorations being unapproached by those of any building hitherto devoted to art in this country. But the accessories of the gallery, splendid as they are, will interest the visitor less than the magnificent collection of national portraits which Mr. Brady displays here for the first time, his labors for the last two seasons in Washington having enabled him to complete the series which he has been so many years forming. The new gallery will be opened for a private view to the press and art connoisseurs of the city on Thursday evening next. Upwards of five hundred invitations have been issued.