In Arcadia, is Thomasina of the Enlightenment era or of the Romantic era?
Although Thomasina evinces elements of both of these eras, primarily her scientific knowledge and her quest for scientific proof demonstrates that she is primarily a character demonstrating qualities of the Enlightenment era. This is shown through her focus on the reason and the intellect as a way of solving various problems and discovering various truths. She is a character who is obsessed by finding equations to explain the natural phenomena she sees around her. Note how this is demonstrated in the following quotation:
If you could stop every atom in its position and direction, and if your mind could comprehend all the actions thus suspended, then if you were really, really good at algebra you could write the formula for all the future; and although nobody could be so clever as to do it, the formula must exist just as if one could.
For Thomasina, even simple actions such as stirring jam into rice pudding becomes an action full of mathemetical possibilities, and her attempt to note and log these actions in order to create mathematical sums based on them shows that she demonstrates more Enlightenment characterists than she does Romantic characteristics.