The marriage on 8th June 1871 at Milton, Glasgow was Alexander Herschel to Mary Thomson,
A fair number of public trees on Anc ...try give this, and it does show (on the preview before paying for the certificate to be opened)that it is Mary Thomson he married on Scotlands People. Only way it can be proved beyond doubt is for the certificate to be opened.
I think there are three Alexander Herschels. One born in Perth, one born in Montrose, and, according to the latest post above, one born in Glasgow.
P.S. I have just seen that certificate, which is on some public trees. It is Alexander Herschel, aged 25, Drapers Assistant marrying Mary Thompson, 23, Dressmaker. Alex's parents: Alexander Herschell & Jane Baldy.