"I see you have a fine Marmor optical bench," he said, "could we use it?"

"Of course," Rosenstrauch replied, always eager to show off his equipment, "here, let me show you a small project I have just started." He gathered together an obscuring pile of books and moved them to a chair, revealing the optical bench resting on a marble surface. The 'bench' consisted of a series of straight wood or metal rods arranged horizontally and held to the marble by a series of clamps. At intervals along the rods were a second set of clamps designed to hold a variety of objects including lenses, mirrors and prisms.

At one end of the 'bench' Rosenstrauch placed a gas burner and covered its flame with an asbestos coated mesh. As the wires in the mesh heated up, the glowing metal gave off a bright white light. A reflector was placed behind the burner so all the light was directed along the length of the bench.

an Optical Bench

"Now we must choose a color," Rosenstrauch said, picking up a series of glass filters of different shades. "Blue, I think. I've been getting good results with this filter. I got it from the Fabrickerfactory only a few weeks ago. It produces almost pure blue light."

He placed the expensive filter in a holder clamped to the nearest rod and right in front of the white light. Then, very carefully, he placed a black plate of metal in front of the blue filter. At first glance it looked as if all the light from the burner and the filter had been cut off, but a closer examination showed that there was a tiny, thin vertical slit in the metal that allowed a narrow band of light to pass through.

"We now pass this ray of blue light through a series of glass blocks," Rosenstrauch said, "and look what happens."

Mendel watched as the glass maker moved a large block of glass into line with the ray of blue light. With the rest of the lights in the room dimmed, Mendel could see that the ray of light struck the surface of the block nearest to the lamp as a thin, tiny thread, and it was possible to see it's direction and path by reflection off the white marble surface.

Rosenstrauch moved the block of glass, turning it slowly one way and the next so that the angle at which the thin ray of light hit it's surface changed. "Here," he said at last, "a perfect 'normal', or at least as perfect as I can get with this equipment."

The faint blue light was striking the surface of the glass block at exactly 90 degrees, or at right angles. "At this angle, and only this angle, the ray passes straight into the glass," Rosenstrauch said, pointing to the way in which the ray entered the glass so that it's path in the air and it's path in the glass were exactly the same.

"That ray of light is what we call a 'normal', a word that probably comes from the Romans and their measuring instruments. Now, if we turn the block of glass even slightly away from this perfect normal, see what happens."

As the glass turned, the thin blue ray of light no longer struck the surface of the glass at exactly 90 degrees, and as Mendel watched, the path of the ray in the air became different from the path in glass. It looked as if the ray of light had a kink in it, and was bent at the point of entry.

"The light is being refracted," Rosenstrauch said. "As the light passes from the less dense medium of the air and into the more dense medium of the glass, its direction of travel is altered in a characteristic way - see!"

Once again he moved the block and Mendel could see reflected from the marble the path of the light develop a bigger and bigger angle as it entered the glass.

"It was Willebrord Snell who discovered the mathematical properties that govern the behavior of light as it moves thus," Rosenstrauch said proudly. Despite his lack of formal higher education he had read widely and understood much of the basic principles that applied to his hobby. He picked up a piece of equipment that looked like the eyepiece of a microscope attached to a prism. "I can measure all the angles of the rays of light using this," he said, placing the vernier eyepiece on a metal scale behind the block of glass.

"The angle made by the ray of light as it enters the glass, as measured from the normal, is called the angle of incidence, whereas the angle made by the light as it travels in the block of glass is called the angle of refraction. It is not easy, and it requires more time than we have now, but I can accurately measure both these angles and repeat Herr Snell's calculations."

Politely, Mendel did not tell his friend that he already knew most of this. Optics had been a major segment of his studies of physics at the University of Vienna. But he was interested in the results the glasscutter had discovered.

"And what have you found?" he asked.

"A most interesting result," Rosenstrauch began, but did not finish, as, at that moment, the two scientists heard the bell ring in the main part of the shop. Herr Rosenstrauch had a new customer.