Lens Making in the 1600s

In the late 1600s, Dutch scientist Antony van Leeuwenhoek made over 500 microscope lenses. He made his first lenses by lamp working, melting and shaping glass over a flame. Heating a rod of glass allows it to stretch into a thin thread. Rotating this thread creates a glass ball. After it cools, this bead is a magnifying lens usable as a microscope. No lenses of this type survive. Van Leeuwenhoek didn't think much of these lenses and soon tried a different method. Van Leeuwenhoek's most common method for making microscope lenses involved grinding and polishing tiny pieces of colorless glass. He used resin or glue to attach a glass shard onto a stick. Grinding the glass shapes it to the curvature of a mold. A gritty material, usually some kind of sand or mixture of grainy ores, removes tiny particles of glass as the mold rotates. A succession of finer powders shapes the glass. Its curvature is regularly compared to another mold or stencil. We don't know how many different grits Van Leeuwenhoek used. Sometimes just grinding was enough to produce a completed lens. Other times the finest grit sand or even putty was put on a piece of felt or leather to polish the lens and remove surface irregularities. Most of Van Leeuwenhoek's surviving lenses are double convex, ground and polished on both sides. Van Leeuwenhoek's letters mention an improved but still secret method of making lenses by glass blowing. This technique probably begins by rotating a thin-walled glass tube over a flame to soften it. Stretching the glass creates a tapered point. Repeating this process leaves a small capsule between two tapered points. Closing one end creates the beginning of a bubble. Heating the capsule and then gently blowing creates an almost spherical bulb. Removing the excess glass leaves a tiny lens with a high magnification. Only two of these lenses survive.