Having found a digitized copy of Records of the Scottish Volunteer Force 1859-1908,
by Major-General J. M. GRIERSON, C.V.O., C.B., C.M.G., I can say with some degree of confidence that this soldier is from the 1st City of Edinburgh Royal Garrison Artillery Volunteers. This is because the photo, taken in 1885 or later, shows the man wearing the busby which had been replaced four years earlier in all Scottish artillery volunteer units, except the 1st City of Edinburgh Royal Garrison Artillery Volunteers, per the extract below:
"As a head-dress, the busby of the Royal Artillery was universally adopted in the 'early sixties,' and this was replaced in 1880-81 by the helmet, at first worn with a spike and afterwards with a ball. At the 'Coming of Age Review' in 1881, the 1st Edinburgh (City) and the 1st Renfrew and Dumbarton Artillery were the only corps which still wore the busby. The latter gave it up shortly afterwards, and the former was the only corps in Scotland which, till 1908, wore the headdress it assumed on its first formation."