This is an ambrotype in an inner pinchbeck casing, measuring 5.1 x 6.1cms, presented in an outer card and cloth casing held shut by a small hook and pin. This would date to the late 1850s or early 1860s. The inside of the lid is padded and embossed.
The first cdv below is of the style of the early 1860s, and shows a very realistic looking window. However, there is little light on the figure from that side, and the view outside doesn't much resemble St. Enoch's square, even in the 1860s! The second shows more development of the design on the back, and probably comes from the mid 1860s.
The design on the back of the cdv below was used by others such as Peter Low who ceased trading in 1867, suggesting that the Misses Gilmour posed for their portraits on or before this date.