How do “night” rear view mirrors work? One flick of the switch and it seemingly dims all. –Chris Gaffney, Toronto

This isn’t going to be easy, but what the hell, a man’s gotta do what a man’s gotta do. Here’s the simple explanation: in a dimming rear view mirror you’ve got two reflecting surfaces–one with high reflectance, one with low. During the day you use the high reflector. At night the dimmer switch swings the low reflector into place, dimming glare from headlights behind you. Satisfied? Fine, you wimp. The rest of you keep reading.

The trick is that the two reflecting surfaces are the front and back of the same piece of glass. Said glass is specially ground so that the back surface is slightly tilted relative to the front one. (In other words, the glass looks wedge shaped from the side.) The back surface is coated with silver, like a bathroom mirror, making it highly reflective. The front surface isn’t coated, but it’s still slightly reflective, just as all glass is.

Because the two surfaces aren’t parallel, any time you look at the rearview mirror, you’re seeing two different reflections simultaneously. During the day with the mirror tilted into the normal position, the silvered surface shows you the road behind you. The nonsilvered surface, meanwhile, shows you the car’s backseat (in GM cars, that is; other makes vary)–but it’s so dim you don’t notice.

At night the situation is reversed. When you flip the dim switch, the silvered surface tilts so it’s showing you the car’s ceiling, which is so dark you don’t notice it. But now the non- silvered surface is showing you the road. Because the headlights of the cars behind you are so bright, the nonsilvered surface reflects enough light to let you see what’s behind you, but not so much that you’re blinded.

The folks at GM tell me that on Cadillacs you can now get a high-tech “electrochromic” mirror that dims at night automatically, without having to flip a switch. The Caddy mirror has only one reflective surface, but there’s a special film in front of it that gradually darkens at night through the magic of electronics. That’s great, but for sheer ingenuity you can’t beat the tilted surfaces. My regards to the inventor–such a relief to know I don’t have to think of everything.


Just a few comments on past Straight Dopes:

1. Pre-Charmin toilet paper substitutes (cf. More of the Straight Dope)–my parents tell me peach season was considered a real treat in the old days because the semi-soft paper each peach was wrapped in was far preferable to the Sears catalog or a corncob.

2. Collecting semen from bulls for artificial insemination [July 28]–I used to work at a vet clinic where we did fertility checks on bulls. We used an apparatus known as an electro-ejaculator that was basically the size of a man’s arm. You inserted it into the rectum (of the bull, silly) and turned it on to achieve the desired results. Not a pretty picture, but I swear it’s true. –Chris Voorhees, Studio City, California


In the discussion in your book on human penis length, you stated, “Specimens as long as 13 inches (when erect) . . . have been reported.” Well, records are made to be broken–or in the case of the enclosed photos, pulverized. This gent beats the old mark by a good five inches. If you think the photos are retouched, altered, or in any way exaggerated, videotapes of this guy in action are readily available through commercial suppliers. –Roger Jackson, West Bend, Wisconsin

The stuff I get in the mail these days is just unbelievable. The fellow in the pictures goes by the witty moniker of Long Dong Silver (no, he’s not Oriental). Though no official measurements are supplied–pornmongers just have no grasp of science–18 inches appears to be in the ballpark.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.