1887.] NEW YORK ACADEMY OF SCIENCES. 95- 



According to another version, Lippersbey^s children were play- 

 ing with the lenses, when one of them, happening to place a 

 convex lens in front of a concave lens, was greatly surprised to 

 see the vane of the clock-tower of the Middleburg Church appar- 

 ently brought nearer. Lippershey^s attention being called to 

 the fact, he tried it, and, working out the idea, he invented the 

 fir.st telescope. 



JMetius, of Amsterdam, the discoverer of the ratio ^ff (the re- 

 lation between the circumference and the diameter of a circle)' 

 claimed to be the inventor. Jansen and Baptista Porta and 

 others disputed for the honor. 



Inasmuch as the first telescopes were at once seen to be of great 

 value in wars, it was attempted to keep the invention a secret. 

 Galileo heard, through letters, that an instrument had been in- 

 vented which rendered distant objects visible, but he obtained 

 no account of the construction. He, however, on this hint, 

 made a telescope after several trials. The highest magnifying 

 power which Galileo used was nearly 30 diameters. He was the 

 first to direct the telescope heavenward. He saw the spots on^ 

 the sun, the moons of Jupiter, the mountains in our moon, the 

 handles of Saturn, the phases of Venus, and made other inter- 

 esting discoveries. 



Kepler suggested for the single biconcave lens near the eye, 

 iTsed by Galileo and others, a double convex lens, which gave a 

 larger field. This combination is called the " Astronomical 

 eye-piece." It inverts the objects looked at. 



It is foreign to my purpose to enter into the details of the 

 construction of a telescope. You all know that the power of a 

 telescope to magnify an object looked at depends npon the focal 

 lengths of both object-glass and eye-piece. It is the ratio of the 

 first to the second. If, then, our object-glass forms an image of 

 the moon at a distance of 100 inches from the centre of the 

 glass, and we view that image with an eye-lens whose focal 

 length is one-quarter of an inch, we obtain an image in the field 

 of view which is magnified 400 diameters. We can, therefore, 

 increase our magnifying power either by making the focal length 

 of the object-glass greater, or that of the eye-lens less, or by 

 doing both. With a given object-glass we can, theoretically,. 



