Constantinople is a universe containing an immensity of Christians. They reckon at least two hundred thousand Greeks, and the Armenians perhaps are eighty thousand, amongst the permanent inhabitants, omitting those that come and go, whom commerce and the court circulate. The plague affords a striking proof how populous this city is. I have seen, myself, two hundred thousand people mown down by the mortality. The dead that passed the portals to be inhumed outside the city, gave us the means to make this calculation. And yet the streets were as full of living people a few weeks subsequently, as if nobody had died.
Many families that inhabited the town in the time of the Genoese, are still at Pera and Galatea ; they amount to about four hundred persons ; they are the ambassadors' interpreters ; while some are doctors, a profession from which they derive consideration, and free admission to the Turkish lords, without even excepting the seraglio. The retinue and households of the several ambassadors and the merchants of then nations, in all almost three thousand persons, are the most distinguished of the Christian Franks. The crowd is increased by the Christian vessels, and the streets near the shore are often to be seen thronged with their newly-landed. The Catholic slaves of Constantinople, who are working the ships, or chained to the gallies, or imprisoned in the bagnio of the Grand Seignior, are 5,000; there are 20,000 others in possession of his subjects. A native of Ragusa, M. R. Galani, a Dominican, and titular archbishop of Ancyra, a gentleman of great regularity of life, and who is strictly attentive to his duties, is ecclesiastical superior to all these Catholics. The situation of our house is such, that we can easily assist this population. At the centre of Galatea, near the sea, in the thoroughfare of all that come from the bottom of the harbour, appears the finest church in Turkey. The pillars that support, the balustrade that terminates the vestibule, and hems the staircase that conducts to it, these are all of snowwhite marble. It possesses the prerogative of mosques, a cupola, and a covering of lead. The nave is decorated by sepulchral monuments of ambassadors of France and of Princess Tekeli; you will find her mother's mausoleaum in a separate chapel. This pious and courageous princess died at Nicomedia. To render her the services which for several years she had received at Constantinople, the Jesuits considered it a duty to visit Nicomedia as long as she resided there. A mission had commenced at Nicomedia, which, impracticable without a plausible pretext, such as a visit to this princess, expired when the lady died.
Link (here) to the full 18th century letter by Father Trallion, S.J.