A letter from Herr Leibnitz set him wondering uneasily what had taken the young German Crichton from Frankfort, and what he was about in Paris. They had had many a discussion in this little lodging, but he was not yet sure of the young man's single-mindedness. The contents of the letter were, however, unexpectedly pleasing. For it concerned not the philosopher but the working-man. Even his intimates could not quite sympathize with his obstinate insistence on earning his living by handicraft--a manual activity by which the excommunicated Jew was brother to the great Rabbis of the Talmud; they could not understand the satisfaction of the craftsman, nor realize that to turn out his little lenses as perfectly as possible was as essential a part of his life as that philosophical activity which alone interested them. That his prowess as an optician should be invoked by Herr Leibnitz gave him a gratification which his fame as a philosopher could never evoke. The only alloy was that he could not understand what Leibnitz wanted. "That rays from points outside the optic axis may be united exactly in the same way as those in the optic axis, so that the apertures of glasses may be made of any size desired without impairing distinctness of vision!" He wrinkled his brow and fell to making geometrical diagrams on the envelope, but neither his theoretical mathematics nor his practical craftsmanship could grapple with so obscure a request, and he forgot to eat while he pondered. He consulted his own treatise on the Rainbow, but to no avail. At length in despair he took up the last letter, to find a greater surprise awaiting him. A communication from Professor Fabritius, it bore an offer from the Elector Palatine of a chair at the University of Heidelberg. The fullest freedom in philosophy was to be conceded him: the only condition that he should not disturb the established religion.