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Please can someone explain this?  I'm an experienced periscopic sextant user, now retired, using a marine sextant (Hughes Mates 3 Circle No 25410) for the first time.  I'm getting quite good results shooting the sun centre to centre using a tray of engine oil as an artificial horizon.  I thought I'd try limb to limb, but I'm unsure about how to apply semi diameter correction when using an reflecting artificial horizon.  Pointing the sextant staight at the sun (i.e. Hs =0 degrees), if I go from UL of the index mirror sun kissing LL of the horizon glass sun to LL of the index mirror sun kissing UL of the index glass sun, I go though 64' on the sextant arc.  If I do the same using my artificial horizon, I apper to need to move the same amount on the arc.  Why don't I need to travel though 128'?  The sun was low; the mirrors are small; the edges of the tray were high; it was windy; and I was sitting on cold hard concrete, so conditions weren't perfect.  Therefore, it might have been finger trouble, but surely not 64’ worth.  I couldn't use a flat glass cover, because the reflection off the glass was blotting out the reflection off the oil. Dave