The nineteenth century saw a great rise in interest in history, which was probably a Good Thing.
On the other hand, it also saw a lot of slipshod, nationally-biased work where authors either made shit up, or were very careful with what they selected.
Nineteenth century historians were very big on ideas like the March of History towards Progress, and thus tended to promulgate ideas like the Renaissance.
Generally, if it's a nineteenth century work, don't use it ... unless (a) you really know what you are doing, or (b) it's a nineteenth century reprint of a historical document.
While it is not unknown for historical documents to have been fabricated in the 19thC, you are more likely to be on solid ground if you use 19th C document collections (eg the Historical Manuscripts Commission collections) than if you use nineteenth century interpretations (eg Burkhardt).
A similar principle applies to using 19th C copies of medieval artworks. The general outline of the picture is usually copied correctly, but the smaller details often have errors which may be misleading (e.g. giving the idea of corsets under bliauts). Ninteenth century copies can give you an idea of whether you might want to look for an original (if it still exists), but often less effort and misconceptions are involved in looking up an original copy in the first place. Many internet sites redistribute 19th century copies of illuminations (without warning about potential errors), because unlike modern accurate copies, 19th C copies are out of copyright.
Basis text Contributed by Anton, 6/11/03. Feel free to correct, redistribute etc