http://members.rpa.net/~choffman/beseler01.htm

I didn't drill into the mirror though.  I just bought 2 $1 mirror tiles.
I scratched off a small area of the reflectance material from one of the
tiles (about the size of a quarter).  This should allows you to see
through the mirror.  Draw an X through the center of this circle.  Put the
unscratched mirror on the baseboard, and the scratched mirror in the
negative stage facing down.  Look through the area you scratched and you
should see the hall effect.

http://www.darkroomz.com/align_your_enlarger.html

Second hit in Google search.

RS

On 3/22/2005 1:51 PM Lew spake thus:

Yep; several places, but the only one I can find just at the moment is
http://members.rpa.net/~choffman/beseler01.htm.

The method is actually very simple: One mirror is cut in a narrow strip that
will be held in the enlarger's negative carrier, mirror side down, sticking
out in front. A small clear spot is either drilled or scratched away so you
can see down through it, with a dark circle drawn around the clear spot on the
mirror. The second mirror is simply laid flat on the baseboard.
(Alternatively, you could put a paper reinforcing ring around the clear spot.
Anything that's visible will work.)

To align the enlarger, simply look down through the hole/clear spot in the top
mirror at the bottom mirror. You'll see a repeated pattern of the dark circle
in a "hall of mirrors" effect. When the enlarger is in alignment, the repeated
reflections will merge together into one image (or close to it, anyhow). This
means that the negative stage and baseboard are parallel.

This method is as accurate as it is possible to align any enlarger. No fancy
store-bought equipment is necessary.

The 2-mirror trick works, it's cheap and it's available at any
hardware store.

I use it and don't like it.  The top mirror has to be held against
the lens barrel with one hand while one peers through the hole in
the top mirror (have the hardware store grind the mirror edges smooth or
pick up some band-aids on the way home) and then using the other
hand turn an invisible (remember, you are peering through the hole
in the mirror so you can't see squat) and hard to get at alignment
screw while leaning over and bent double at the waist.

To make the alignment easier I use cross of two strips of white
correction tape around the hole and illuminate the tape/top mirror
with a Tensor lamp.  The classic method uses a magic-marker black cross,
but it is hard to illuminate black and so the view is quite
dim.

To make the job much easier, Guillermo posted earlier about using
four LED's shining out through the top mirror.  There was a website
illustration but it has disappeared.  Herewith the original r.p.d.
post:

beautifully!!   It is nothing extraordinary but thought to post a
description for the sake of those that like me are looking for a cheap way
to align their enlarger.

holes 1.5" from the center and equidistant from each other, these holes were
just big enough to allow the insertion of  4 standard LED's (one in each
hole), the LEDs were series connected, a 50 resistor was added to the
circuit to limit the current and LEDS+resistor fed by a 9volts battery. The
configuration of holes of the piece of wood was replicated at the end  side
on one of the mirrors, holes were not made on the mirror, instead its
backing was scraped.  The piece of wood with the circuit was glued onto the
mirror.  I happened to have the prism of my defunct very first SLR (Minolta
XG2), I placed this prism on the center hole of the piece of wood and can
see very clearly the alignment dots-cross pattern thru the prism.  A prism
is not absolutely necessary, one can peek from above the mirror and thru its
center hole, but a prism make the alignment so much easier.  A prism can be
easily obtained from a defunct SLR camera, eBay lists many broken SLRs going
for very little money.  Edmund Sci. has a right angle prism for about $10,
this place also sells surplus prisms http://www.anchoroptics.com/ 

I still, however, lust for a Versalab when alignment time comes around.