“The shape of the double convex lens, d, c, is that of two plano-convex lenses, placed with their plane surfaces in contact, and consequently the focal distance of this lens is nearly the centre of the sphere of which one of its surfaces is a part.” -Comstock 1850
J. L. Comstock A System of Natural Philosophy: Principles of Mechanics (: Pratt, Woodford, and Company, 1850) 236
2400×1704, 257.8 KiB
1024×727, 42.8 KiB
640×454, 23.7 KiB
320×227, 9.3 KiB