On the 6th May 1793, Casanova wrote Opiz:  “The letter of recommendation you ask of me to the professor my brother for your younger son, honors me; and there is no doubt that, having for you all the estimation your qualities merit, I should send it to you immediately.  But this cannot be.  And here is the reason.  My brother is my enemy; he has given me sure indications of it and it appears that his hate will not cease until I no longer exist.  I hope that he may long survive me and be happy.  This desire is my only apology.”

“The epigraph of the little work which I would give to the public,” Casanova wrote the 23rd August 1793, “is ‘In pondere et mensura’.  It is concerned with gravity and measure.  I would demonstrate not only that the course of the stars is irregular but also that it is susceptible only to approximate measures and that consequently we must join physical and moral calculations in establishing celestial movements.  For I prove that all fixed axes must have a necessarily irregular movement of oscillation, from which comes a variation in all the necessary curves of the planets which compose their eccentricities and their orbits.  I demonstrate that light has neither body nor spirit; I demonstrate that it comes in an instant from its respective star; I demonstrate the impossibility of many parallaxes and the uselessness of many others.  I criticize not only Tiko-Brahi, but also Kepler and Newton . . . .

“I wish to send you my manuscript and give you the trouble of publishing it with my name at Prague or elsewhere . . . .  I will sell it to the printer or to yourself for fifty florins and twenty-five copies on fine paper when it is printed.”

But Opiz replied: 

“As the father of a family, I do not feel myself authorized to dispose of my revenues on the impulse of my fancy or as my heart suggests.... and no offer of yours could make me a book-seller.”

This shows plainly enough that Opiz, for all his interest in Casanova, had not the qualities of true friendship.

On the 6th September 1793, Casanova wrote: 

“I will have my Reveries printed at Dresden, and I will be pleased to send you a copy.  I laughed a little at your fear that I would take offense because you did not want my manuscript by sending me the ridiculous sum I named to you.  This refusal, my dear friend, did not offend me.  On the contrary it was useful as an aid in knowing character.  Add to this that in making the offer I thought to make you a gift.  Fear nothing from the event.  Your system of economy will never interfere with either my proceedings or my doctrines; and I am in no need of begging you, for I think that your action followed only your inclination and consequently your greatest pleasure.”