Madame L chanced upon this word in "Aunt Dimity and the Duke," by Nancy Atherton.*
In this book, a reticule is a small handbag, with netting and a drawstring, decorated with embroidery or beading (which is Google's definition of the word). Ruth and Louise Pym, 90-something-year-old twins whom the heroine of the story meets early on, carry reticules.
Google also provided Madame L with a plethora of images of reticules, such as this one.
Madame L loves the very idea of such a reticule and hopes you, her Dear Readers, will do a Google image search to see some of the many lovely variations of reticules.
Madame L had heard the word in another context, though, so she looked it up to be sure, and, sure enough, a reticule is also the "network of tiny lines that make up a sighting device's eyepiece" (which is the Vocabulary.com definition of the word). These could be in a gun scope or sight, a telescope, or a microscope.
Madame L was fascinated to learn that the word reticule, which is also sometimes spelled "reticle," comes from the Latin "reticulatus," because of the net-like pattern, which comes from the Latin "reticulum," or "little net."
Madame L would love to have read a spell like "Reticulatus!" in the Harry Potter books resulting in a spider-web-like net being thrown over a bad guy. Even better, she would love to read a book in which little old ladies like Ruth and Louise Pym are carrying their dainty little reticules while looking through a telescope's reticule.
*Madame L will write a review of this book, anon ("soon, shortly"). Madame L is inspired to use words like "anon" when she reads words like "reticule" in books like "Aunt Dimity and the Duke."