The Brown Argus butterfly is a relative of the small blue butterflies. Male and female Argus are similar; orange border spots on topside of female reach to leading edge of wing but those of male are 2-3 spots short of wing edge. ("Northern Brown Argus", "Plebeius artaxerxes", has few/no orange spots on topside of forewings.)
The Brown Argus sometimes has a blue sheen on the hair near the body but does not have any blue scales, unlike the similar female Common Blue which may have blue scales. The Argus also has a stronger black mark in the centre of the forewing than the female Common Blue.
The Brown Argus looks very similar to the female Common Blue from the side (latter has more blue scales near the body and a slightly different pattern of spots).
The two spots circled on the hind wing are closer together on the Argus; the empty circle on the forewing has a spot on the Common Blue.