Hebrews 10:1

Heb. 10:1. For the law having a shadow of good things to come, (and) not the very image of the things, can never with those sacrifices, which they offered year by year continually, make the comers thereunto perfect.

In the following comment we find the author of Images and Shadows of Divine Things defining shadows as "dark resemblances" which are nevertheless clear enough for their purpose.

[288] Hebrews 10:1. "The law having a shadow of good things to come, and not the very image of the things." Here a shadow is distinguished from images, or pictures, as having a more imperfect representation of the things represented by it. The types of the Old Testament are compared to this kind of representation of things, not only here, but chap. 8:5, and Colossians 2:17, which fitly resemble them on several accounts. The shadow of a thing is an exceedingly imperfect representation of it, and yet has such a resemblance, that it has a most evident relation to the thing of which it is the shadow. Again, shadows are a kind of dark resemblances. Though there be a resemblance, yet the image is accompanied with darkness, or hiding of the light: the light is beyond the substance, so that it is hid. So was it with the types of of the Old Testament; they were obscure and dark; the light was beyond the substance. The light that was plainly to reveal gospel things came after Christ, the substance of all the ancient types. The shadow was accompanied with darkness and obscurity; gospel things were then hid under a veil.

In "Concerning Faith" Edwards applies this principle to the Old Testament.

It was of old the coming to the sacrifice, as one consenting to the offering, active in choosing and constituting that as his offering, and looking to it as the means of atonement for his sins, that interested him in the sacrifice; as appears by Hebrews 10:1; Hebrews 10:2; "Could never make the comers thereunto perfect. For then, the worshippers once purged should have had no more conscience of sins." Compare chap. 9:9.

Heb. 10:2-4