The first spyglass was a monocular or a small hand-held telescope.  The children of an optician named Johannes Lippershey of Middleberg, Netherlands, first discovered the telescope in 1603.  The children were playing with lenses when they discovered that by using both a concave and convex lens together, they could see a far away belfry.  In 1608, Lippershey applied for a patent of the telescope, but was denied. 

The type of telescope Lippershey and others developed could magnify an object up to 30 times its normal size and was generally called a refractor telescope.  These telescopes were long and not ideal for spying. 

In 1680, Sir Isaac Newton developed the reflector telescope, which replaced one of the lenses in a typical telescope with a mirror.  This development reduced the tube length of telescopes to a more manageable size.  At the same time, tubes were made with sliding sections that allowed the telescope to collapse.  Eventually, these innovations helped create the smallest spyglasses, allowing people to place them in any number of everyday objects.

Size was not the only technical problem for spyglasses.  Early lenses bent light at slightly different angles, causing the viewer to see a blurry rainbow around the edges of an image.  This phenomenon, known as chromatic aberration, was solved by Chester More Hall in 1730.  Hall used two lenses together, one made of old crown glass and the other of newer flint glass, to correct the problem.  John Dolland heard of these findings and manufactured the first achromatic lens in 1760.  An unforeseen benefit of the achromatic lens was their larger diameter, which created greater magnification.