Begin the Beguine

Item A: In Amsterdam there is a Begijnhof, where beguines, women belonging to a lay religious order active in Germany and the Low Countries between the 13th and 16th centuries, once lived. According to Wikipedia: Beguines were not nuns; they did not take vows, could return to the world and wed if they chose and did not renounce their property. The last Amsterdam beguine died in 1971, and today the building is a historical landmark.

Item B: Cole Porter composed the song Begin the Beguine in 1935 and it went on to become a popular hit.

I first encountered the song, and much later the Begijnhof in Amsterdam. I thought it was mere coincidence. What did a religious order from the middle ages have in common with a 20th century secular song? But recently I discovered there indeed was a connection. Wikipedia explains that in the creole of Caribbean, especially Martinique and Guadaloupe, beguine came to mean a white woman, then a style of music and dance, then used as a style by Cole Porter. He did not invent the song form but was indeed responsible for popularising it.

Begin the Beguine is unconventionally long at 108 measures, far longer than the normal 32 bar AABA form. Its score takes up 6 pages in my song book. It is a delight to play but difficult to learn. Besides the lovely modulation, there are also tricky variations in phrasing in similar measures, and learning the lyrics as well helps recall. It is a testament to Cole Porter's songwriting genius.

Being the prototypical beguine, it is perfectly matched to that setting of a digital keyboard or piano. Other songs that go well with this setting are Leroy Anderson's Serenata, Tonight from West Side Story, and Misirlou, another song with a fascinating history.

What is the power of my spectacles?

I needed to know the power my reading glasses to shop for spares. Problem was, I wasn't sure of the exact figure and of course the original sticker had been peeled off long ago. How to find the dioptre without measuring equipment?

Then I recalled the thin lens formula from high school physics:

For a distant object, that term becomes small so for a convex lens the image comes to focus approximately at the focal length.

So I held the spectacles in front of a wall, covering up one lens to avoid double images, and measured the distance at which distant scenery in the window came into focus. I arrived at 0.4m. The dioptre of a lens is the reciprocal of the focal length in metres so the answer was 2.5. That agreed with my recollection of a sticker reading +250.

This procedure doesn't work for concave lenses used for correcting myopia.

Incidentally optometrists state lens power in dioptres because this measurement has the convenient property that the dioptres of two adjacent lenses add, to a first approximation. Sometimes after an eye exam the optometrist will put a thin correcting lens, e.g. 0.25 dioptres in front of your spectacles to show you it's clearer, and so you need to change your next prescription by that much.