Very rare and almost perfectly conserved 11 page treatise on the comets C/1664 W1 and C/1665 F1. The author, father Jacques Grandamy (1588-1672), a french Jesuit, theologian, philosopher, physicist and astronomer, tried to decide on the the question whether both comets were identical or not, since they looked quite similar and appeared shortly after another. Although he maintained that the latter comet looked brighter with a longer tail he was not able to come to a final conclusion.

Besides the text, and the tables with the positions of the comets the treatise contains two large copper engravings showing the apparent path of the two comets. Size is 39 cm x 18 cm each.