The Mentor · The Story of the Lens · Page 2

application of glass to spectacles. The eminent American optician, Mr. John A. Brashear, places the date of this discovery as being "somewhere between 1280 and 1311." Preserved in an Italian library is a manuscript written in 1299 in which occurs this sentence: "I find myself so pressed by age that I can neither read nor write without those glasses they call 'spectacles,' but lately invented."
To whom belongs the credit for first having used lenses in some form of optical instrument has never been settled. As early as 1590 a Dutch optician,

OPTICAL SYSTEM OF GREGORY'S
REFLECTING TELESCOPE

Zacharias Jensen, placed convex and concave lenses at the ends of a tube about eighteen inches long and used the combination for the purpose of magnifying small objects.

OPTICAL SYSTEM OF
GALILEO'S TELESCOPE

A fellow countryman, Johannes Lippershey, a few years later experimented with a similar arrangement of lenses and made the discovery that a distant church steeple was brought much nearer by their aid. But to the genius of Galileo Galilei, Italian musician, scholar, teacher, physicist, inventor and astronomer, the world will forever be indebted for the invention of the astronomical telescope, an instrument that has extended the bounds of human observation by millions of miles. Galileo's Telescope Galileo's telescope, or "Optical Tube," as he called it, consisted of a lead tube in one end of which was a double convex object glass, and, in the other, a double concave eyeglass.

OPTICAL SYSTEM OF
CASSEGRAIN'S REFLECTING
TELESCOPE

With this first instrument he brought objects three times nearer and made them appear nine times larger. He quickly made other glasses, each of higher power than the preceding, and, in a short time, had a telescope that brought objects thirty times nearer than when viewed with the naked eye. With this instrument Galileo made his epoch-making discoveries in astronomy. To his amazement he found that he could count ten times

SYSTEM OF LENSES IN
TERRESTRIAL TELESCOPE

as many stars as his unaided eye was able to detect. Contrary to the common belief, then, the stars were not all equidistant from the earth. Those that were brought into view with his telescope, he concluded, must be at greater distances than those seen without its aid. He next turned his magic tube on the