An elegant hat (ca.1850)

The Royal Victoria Knitting Book : containing an assortment of the most elegant patterns for hats, hoods, feathers, and bonnets, published in London by W. Clark, possibly in 1850, gives patterns for some very charming knitted hats and bonnets indeed.  The Victoria Knitting Book was published in installments; the copy at the Internet Archive linked above contains pages 85-96 (breaking off right in the middle of a pattern!); the full book can be seen here, courtesy of the Richard Rutt Collection at the University of Southampton.  Other installments focused on lace edgings and insertions, cuffs and collars, doilies and antimacassars, polkas, stockings, shawls, etc.

Here is the pattern for the exquisitely lacy hat No. 40, at top in the image above --

The "curtain" is the part of a bonnet or hat that covers the back of the wearer's neck.