Hi Bill,
Is the micrometer usable on a modern telescope?

Tim,

Short answer, ‘yes’, long answer, ‘if you can get it to fit‘. This particular example has an eyepiece fitting slightly larger than 2” in diameter, so not easy unless you have custom fitting. Plus these micrometers can have a focal plane where it’s difficult to wind the focuser in far enough to get a workable image. These days there are ‘astrometric reticule’ eyepieces available which will give you most of the function of a micrometer if you want to pursue this line of enquiry. 

Hi Bill, Nice piece of kit. Are the webs still intact, if so are they spider webs or has someone rewebbed using modern filaments?

I have heard that the best modern filament can be obtained by unwinding dental floss down to single strands. Have you ever rewebbed a micrometer?

Another question, is there provision for focal plane illumination or was there bright field illumination on the McLean telescope?

Denis, yes, the webs are still intact, but are not in good condition. I’m guessing they’re the original spider. No I’ve never re-webbed an instrument, I’ll bear in mind the dental floss if I ever do.

Denis, there are the remains of what appears to be a field illuminator (filament lamp or LED) on the underside.