Meanwhile the production of refracting telescopes made but slow progress.  As late as 1836 the largest instrument of this kind in the world was the eleven-inch telescope of the observatory at Munich.  The next in importance was a nine and a half-inch instrument at Dorpat, in Russia.  This was the telescope through which the astronomer Struve made his earlier studies and discoveries.  His field of observation was for the most part the fixed and double stars.  At this time the largest instrument in the United States was the five-inch refractor of Yale College.  Soon afterward, namely, in 1840, the observatory at Philadelphia was supplied with a six-inch refracting telescope from Munich.

German makers were now in the lead, and it was not long until a Munich instrument having a lens of eleven inches diameter was imported for the Mitchell Observatory on Mount Adams, overlooking Cincinnati.  About the same time a similar instrument of nine and a half inches aperture was imported for the National Observatory at Washington.  To this period also belongs the construction of the Cambridge Observatory, with its fifteen-inch refracting telescope.  Another of the same size was produced for the Royal Observatory at Pulkova, Russia.  This was in 1839; and that instrument and the telescope at Cambridge were then the largest of their kind in the world.

The history of the telescope-making in America properly begins with Alvan Clark, Sr., of Cambridgeport, Massachusetts.  It was in 1846 that he produced his first telescope.  Of this he made the lens, and such was the excellence of his work that he soon became famous, to the degree that the importation of foreign telescopes virtually ceased in the United States.  Nor was it long until foreign orders began to arrive for the refracting lenses of Alvan Clark & Sons.  The fame of this firm went out through all the world, and by the beginning of the last quarter of the century the Clark instruments were regarded as the finest ever produced.

We cannot here refer to more than a few of the principal products of Clark & Sons.  Gradually they extended the width of their lenses, gaining with each increase of diameter a rapidly increasing power of penetration.  At last they produced for the Royal Observatory of Pulkova a twenty-seven-inch objective, which was, down to the early eighties, the master work of its kind in the world.  It was in the grinding and polishing of their lenses that the Clarks surpassed all men.  In the production of the glass castings for the lenses, the French have remained the masters.  At the glass foundry of Mantois, of Paris, the finest and largest discs ever produced in the world are cast.  But after the castings are made they are sent to America, to be made into those wonderful objectives which constitute the glory of the apparatus upon which the New Astronomy relies for its achievements.