fixing up scratches in brushed aluminum  

Do you know anyone with a...
Do you know anyone with a polishing wheel ?

Would a metal factory (welding, lasercut, stamp, extrusion)
have this sort of machine?

The factors
to keep in mind are, A) everything you do to the surface with a tool different from the original one will create a different effect, and B) scratches differ widely in their depth, and no final result will be acceptable if the deepest scratches are not removed. Therefore, the initial work must be to go deep enough to reach the bottom of those scratches.
This implies that a coarser grade of abrasive should be applied first. Only after that application has created a uniform appearance to the damaged area should the next finer abrasive be employed -- whose purpose is the remove the marks of the previous abrasive. Step by step, finer and finer grits or grades are employed. For a perfect appearance, the exact same technique as originally used must be re-created. Over-polishing will be as disappointing as would a texture which was too coarse.
Thus, a practice piece, of similar metal, should be employed during the re-surfacing procedure.
If this fixture cannot be disassembled into separate parts, I'd tend to advise that no attempt be made to fix the damage.

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varnished ?
sometimes aluminium was varnished ! in that case remove varnish and re- spray them !

bj
Killian's advice is on the right track. There is a thriving industry that specializes in restoring architectural metals; from doors, radiator cabs, to light fixtures. For a relatively small object like your light fixture you could try a shop that specializes in door hardware restoration

Thanks to all!
It's all good advice, it seems. I'm going to go with uasrem tip first (see if it is possible to find such a thing in my area), otherwise I will try a metal workshop locally (if they can produce good results, as sdr states) if not I will buff it out as Jim advises.
One more thing, on the comment of it possibly being varnished: were these raak lamps varnished??
Thanks!

BJ,
Any antique dealer...
BJ,
Any antique dealer will point you in the direction of a good polisher. I have my own polishing wheel at work and in my opinion any person who polishes brass could polish your lights....its actually quite easy and quick and based on my experiences its also cheap. Look for either a local brass polisher or an antique dealer who sells lots of shiney brass things.

possibly lacquered ?...easy...
possibly lacquered ?...easy to find out....get some metal cleaner ( I like a product called Peek ) and polish / rub a scratched area along the scratch...keep rubbing and the varnish will start to disappear in a way you will immediately recognise as being disappearing varnish rather than metal being clean....it will become apparent if its there....if its just bare metal then everything but the scratch will start to get clean.

It should be
apparent that these lamps are not just "polished." Polishing is the process of bringing the metal to its maximum state of smoothness, so that it becomes essentially reflective. These lamps are described as "brushed" by the owner; once a texture other than "polished" is present, the kind of brushing becomes an issue, if a perfect replication is to be achieved.
The reason I inquired about dismantling the affected fixture is that (as can be seen) the whole surface of each curved segment is not accessible in any other way. Applying the middle segment (the damaged one) to a brushing or polishing wheel (or vice versa -- applying the wheel to the metal) will be hindered by the proximity of the adjacent segment, resulting in an incomplete and thus uneven re-surfacing.
One good way to achieve a restored and uniform appearance is to re-texture all six segments of the pair of fixtures -- stripping and re-brushing the segments, after the damaged one has been cleared of its cross-scratches. As in woodworking, only when the irregularities are canceled out are all pieces ready for a uniform sanding and finishing.
As for determining a finish, it would most likely be a lacquer-based clear coat, wouldn't it ? This is checked more easily and with less potential damage by subjecting a small corner of the metal to some lacquer thinner -- rather than grinding through the surface in the middle of the most visible part of the fixture !

I agree, SDR.
I dismantled the thing yesterday, so that I am now left with only the (separate) tubes.