Right after seeing Tim's Vermeer, I placed a hold on David Hockney's Secret Knowledge at the library. It came in this week, and I stayed up too late three nights in a row reading it. It's a fascinating analysis showing how the Old Masters used converging lenses and mirrors to transform the three dimensional world into a two dimensional image in the same way a camera does today, with their brushstrokes capturing the image in place of the film or electronics we now use. The reason artists like Vermeer, van Eyck, Caravaggio, Velázquez, and numerous others were able to create photographic likenesses in their portraits so long ago was that they were actually doing their own kind of photography by letting the light focused by the lens or mirror graph the spatial relationships for the picture. Though the written hints of how they did this are rare, the evidence in the paintings is clear – the optical distortions of a lens or mirror that aren't present when our human pair of eyes look directly at a room or a face have been faithfully reproduced. It's a fascinating book, and David Hockney's writing is full of the delight of discovery.

Reading Secret Knowledge also brought back a favorite memory from teaching a high school physics class in Indonesia. For one of the labs, we used convex lenses (they're thicker in the middle, and able to focus light) to show the physical basis for the ray diagrams in the textbook. It was fun to see the dim image of the wavering candle flame change position and magnification as we moved the lens towards or away from the actual candle, but the real magic happened when we held the lens up to the big windows along one wall of the classroom, and held an index card at the focal point on the opposite side – a glowing image of the distant hills appeared on the paper, upside down, but alive with the movement of clouds across the shimmering blue sky.

You can get the same sort of image from a pinhole in a darkened room (camera obscura) or with a concave (magnifying) mirror, and it's certainly worth a try if you have a convex lens or a concave shaving mirror handy, or an interesting view out your window and good curtains. There happens to be a makeup/shaving mirror in the bathroom, so here's the image of the kitchen tile and cabinets, projected onto paper. Magic.