The officer pictured is wearing the oak leaves of a major.

I subscribe to Shorpy in my Google Reader and I've been behind for quite some time. Just a few days ago I was catching up on the posts and I remember seeing this image. Anyway, fast forward to today and I've been reading a book about solar history and I was doing some research. I was using Google Books because they just recently added some periodicals to the archive, some of them being Popular Science and Popular Mechanics. I was looking at the October 1916 issue of Popular Science Monthly pages 515-518, then I scrolled down to page 519 to see if the article continued at all and what do I see before me?! This exact image, so it's not from 1918, but is from at least from October 1916.

I used to think of coincidences as just odd things, something to share as a story of an odd experience. For the past few years though, I've changed my ideas about coincidences as being opportunities to discover something, or somehow how the universe is giving you every chance to make a connection. What's the connection? I have no idea, that's for me to find out. It's just extremely odd that I see this random image on Shorpy, which there's no other way I would have seen it otherwise. Then I just happen to be doing some research while reading an old book from 1980, when the book mentions some of the research in better detail in Popular science from 1916. There would be no way for me to see this issue, but it just so happens that Google has just added Popular Science to it's Google Books archive. I took advantage of that archive, then just happen to scroll down to the next page after the article and see this exact image that I just saw for the first time not a few days ago. I don't know, way too many variables to make this a coincidence, but why? Maybe one day I'll find out.

Here's where I found the image...
http://books.google.com/books?id=0ygDAAAAMBAJ&printsec=frontcover#PPA519...

[Very interesting. As for when the picture was taken, "circa 1918" means around 1918, not necessarily that exact year. Thanks to your comment we can make it circa 1916. - Dave]

U.S. Public Health Service.

This looks like a variant of an ophthalmometer: a device to measure curvature and other irregularties of the cornea. Originally invented by German physicist Hermann von Helmholtz, the cumbersome table-mounted instruments have largely been replaced by hand-held ophthalmoscopes.

Can anyone interpret the insignia on the optometrist's uniform?

Those are heavy-duty freckles. I've had those up to a few years ago (I'm 58 now), and when I went in the sun for a few hours, I came back looking like this -- with freckles on my lips even! Strange, but better than acne.

I like the old-school charts. No big E, on the top, we're doing the big O here!

It's unusual to notice the number of spots on a person, but a.) freckles seem rare on 100-year-old photographs of grown men, and b.) the optometrist has got a ton. Or are those liver spots?

There's something very sinister about this picture. Maybe it's the electrical cord that's hooked up to the light. He's going to extract the information he wants from this poor girl one way or another.

and tell me what you see ...

[Retinal capillaries. - Dave]