It is therefore not very difficult to produce a usable eyepiece.
The kago was the humblest of all conveyances recognized as usable by the upper classes.
Rapid settlement, excessive prices, reckless waste of money, and wild commercial ventures that glutted San Francisco with all objects usable and unusable made the following years astounding from an economic point of view; but not less bizarre was the social development, nor less extraordinary the problems of state-building in a society " morally and socially tried as no other American community ever has been tried " (Royce).
By experiment Abbe proved that old, good microscope objectives, which by mere testing had become so corrected that they produced usable images, were not only free from spherical aberrations, but also fulfilled the sine-condition, and were therefore really aplanatic systems.
Aqueduct (1831-1835) is still usable in case of need; its supply was the Almendares river (until long after the construction of this, a still older aqueduct, opened at the end of the 16th century, was in use).