Does one "douse" or "dowse" a spotlight or projector?

Dowsing means divination of underground resources (water, oil, buried treasure, etc.), usually by means of a forked stick. I rarely if ever hear about it here in Los Angeles, but when I lived up in the hills, our neighbor had the reputation of being an excellent "water witcher".

Dowse is given as an alternate spelling of douse (to put out or extinguish, as fire or hope) in several dictionaries, as @tchrist has pointed out - but in 21st-century usage, it is highly unusual.

Per the OED, dowse is a variant spelling of both douse verbs. They do not know where the water-seeking verb came from. I’ll give the nouns first.

dowse, n.¹ [a 1627]

Pronunciation: /daʊs/
Forms: Also 16 douze, 16– douce, dowse, 18 douss.
Etymology: < douse v.¹

    A dull heavy blow or stroke.

† douse | dowse, n.² [c 1325 / a 1500]

Etymology: perhaps subst. use of douse, douce adj. sweet.
Obs.

    A sweetheart; a ‘dear’. Also ironical.

douse, v.¹ [1559]

Pronunciation: /daʊs/
Forms: Also 16– dowse, 17 dousse.
Etymology: Of obscure origin: known only from 16th cent. In sense 1, perhaps related to Middle Dutch dossen, or early modern Dutch doesen to beat with force and noise (Kilian): compare also East Frisian dossen to beat, strike, punch, knock, and German dialect dusen, tusen, tausen, etc. to beat, strike, butt (Grimm). Senses 2, 3 may be the same word; compare ‘to strike sail’; sense 4 is more doubtful, and may be distinct. All the senses belong to the lower strata of the language.

1. † trans. To strike, punch, inflict a blow upon.
2. Naut. To strike (a sail); to lower or slacken suddenly or in haste; to close (a port-hole).
3. To put off, doff.
4. To put out, extinguish, dout (a light).
5. To throw down, table (money): = doss v.¹ 2.
6. To ‘shut up’, stop, cease.

douse, v.² [1600]

Pronunciation: /daʊs/
Forms: Also 16 dousse, dowsse, douze, 16– dowse, douce.
Etymology: Appears c1600: origin unknown; perhaps onomatopoeic; compare souse. ^{_{It is of course not impossible that it arose out of douse v.¹, though connection is not obvious.}}

1. † trans. To plunge vigorously in water, or the like; to immerse with force. Obs.
2. To throw water over; to water, to drench.
3. intr. To plunge or be plunged into water.

dowse, v. [1894]

Pronunciation: /daʊz/
Forms: Also dowze, douse.
Etymology: Derivation unknown; apparently a dialect term.

    intr. To use the divining- or dowsing-rod in search of subterraneous supplies of water or mineral veins.

Derivatives
  • dowsing n.
  • dowser n. /ˈdaʊzə(r)/ one who uses the divining-rod, a water-diviner.
  • dowsing-rod n. the rod or twig used by dowsers.

All these entries are from the online 3^rd edition of the Oxford English Dictionary, but also appeared in the 2^nd edition from 1989, with an earlier version also appearing in the New English Dictionary (that is, the 1^st edition) of 1897.

I've operated a spot lamp in theaters a number of times here on the California side of whatever pond you choose, and if you want to extinguish the light, you "douse" it using the chopper blades, which are operated by a lever, or stick, if you will.

Any reference manual for a spot lamp will use the term "douse" to mean to dim or fully extinguish the light from the lamp.

Here's an example for a well-known brand of spot lamp...

https://www.brockport.edu/theatre/Trouperette3.pdf

I've always thought that the action of dousing a light, as well as the searching for water variant "dowse", referred to the actual act of dipping a stick 'down', either to put out the flame at the end of a torch, or moving a lever to throw a cover over the spotlamp beam, or twitching a stick as you stumble around searching for water or minerals.

The last time I saw anyone 'dowse' for something, it was a septic tank repair man at my in-law's house in the mountains of Southern California. He was unable to locate the main drain line and cap over the tank precisely, and I kept sighting along the six inches or so of drain line that was just visible above ground near the house, pointing off to the north and down the hill where he was dowsing, and I kept telling him "just dig along this line" and pointing, and that's how he finally found the tank.

It was interesting to watch him wander around the property with two bent wires rolling between his fingers, but I saved him (and my in-laws) at least a half-hour just using my knowledge that pipes are often manufactured as straight tubes. : )