There is a dry glitter in his eyes, and a firm set of his thin lips, which goes with a positive and possibly a harsh nature.
First, the bad news. In searching for the phrase "dry glitter" in the Gutenberg e-text, I discovered that Watson has used it once before, to describe Holmes himself. So points must be deducted for repetition, and for applying what I think is a quite distinctive description to two different characters. Also, the "firm set of his thin lips" doesn't really break new ground imagery-wise. The interpretation of how Stapleton's features might relate to his personality should probably not be attempted except in special circumstances such as these. It's classic showing and then telling, which would normally indicate authorial distrust of the reader--but this is what Watson and Holmes do, of course: observe and then interpret. So Watson gets a pass, although he's probably relying on outdated notions of physiognomy here.
Still--that dry glitter! I like this image immensely, perhaps because it reminds me of a snowy field on a sunny January day. It's also (at least to me) quite unexpected. When eyes glitter, it's usually because they are moist with tears, or sparkling with merriment, or gleaming with malice. The dryness is the surprise, and it intrigues Watson enough to set him speculating. If we want to be really generous--and because I haven't finished the story, I can't be sure this isn't true--we could say that Watson, on some level, recognizes the "dry glitter" in Stapleton's eyes as the same one he's seen in Holmes's, and suspects a similarity in character as well. But right now it feels like a mistake. In any case, Watson--and Conan Doyle--clearly see the image as striking, and worthy of close attention.
Back in 2003, Charles Baxter wrote an article for The Believer about the lack of facial description in contemporary fiction, as opposed to fiction from previous centuries. Reading all these "old" stories for my Borrowed Fire series, I've really come to see how true that is. And it's made me frustrated with what seems like my own limited repertoire for describing characters physically: stocky, short, thin, tall, blue eyes, brown hair, glasses... Dull! Without making the kinds of claims Watson makes about the direct relationship of appearance to inner character, surely we all can do better.
Why not spend an afternoon in a cafe surreptitiously glancing at the people nearby and describing their features in our notebooks? Not just their faces, but bodies, gestures, voices. Pay more attention when walking down the street or across campus or through the airport. I'm already thinking of someone I passed on the way to the bank this morning: slightly pigeon-toed, wearing glasses too wide for her face so that her eyes, rather than the lenses, seemed connected by the glasses' bridge... One could even keep a list of these characteristics, to mix and match later.