E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
means to look out of a watch-tower, to spy about (Latin). Metaphorically, to look at a subject with the minds eye, to spy into it; in commerce, to purchase articles which your mind has speculated on, and has led you to expect will prove profitable. (Specularis lapis is what we should now call window-glass.)