Seven reproductions of Scheme XXIV from Robert Hooke's Micrographia, most from academic websites. Images link back to original site.
Robert Hooke is probably best known for coining the term "cell" to describe the ordered substructures he noticed when examining cork bark under a microscope of his own design. He is also remembered for a bitter quarrel with this superstitious lout. A full list of his acheivements also includes describing Jupiter's Great Red Spot, working out the mechanism of petrifaction, and inventing the universal joint.
Each reproduction of the stunning twenty-first plate of Micrographia reveals a slightly different set of details and nuances captured in the original. Note the contrasting moiré patterns. You can explore the image in greater detail here. Hooke described the compound-eye of his "grey Drone-Fly":
The greatest Part of the Head consisted of two large semicircular and regular Protu- berances or Eyes, A B C D E ; the Surfaces of which were covered all over with, or shaped into Multitudes of minute Hemispheres, disposed in a triagonal Order, and in that Order forming exact and equidistant Rows, with little Trenches or Furrows between each.
These Hemispheres were of different Sizes in different Parts of each Eye ; the lower- most Half of them looking downwards, viz. C D E, C D E, being a great deal smaller than the Half A B C E, A B C E, looking upwards, fore-right, sideways and backwards ; a Variety unobserved in any other small Fly. (Hooke 1664)
Hooke also understood the implication of all those slightly askew hemispheres:
these rows were so dispos'd, that there was no quarter visible from his head that there was not some of these _Hemispheres_ directed against, so that a Fly may be truly said to have _an eye every way_, and to be really _circumspect_. And it was further observable, that that way where the trunk of his body did hinder his prospect backward, these _protuberances_ were elevated, as it were, above the plain of his shoulders and back, so that he was able to see backwards also over his back. (Ibid)
Hooke also surmised that insects likely had some mechanism of integrating the information coming from the "Multitudes of minute Hemispheres", just as the humans brain integrates discrete images from both retinas into one composite image with depth.
Current scientific understanding of the optical systems of insects suggest that while they probably do not resolve distant objects as well as human eyes, they are exceptionally adept at detecting nearby motion (as anyone who has tried to swat a fly knows). Additionally many insects are able to distinguish polarity and see UV light, both things that humans have had to invent instruments to do for them.
One of the most amazing things about Hooke's fly illustration is how well it stands up to comparison with the images produced by modern microscopy like the beautiful stuff at Microangela. It's amusing to reflect on the snark of some unknown 17th century critic who described Hooke as "a Sot, that has spent 2000 £ in Microscopes, to find out the nature of Eels in Vinegar, Mites in Cheese, and the Blue of Plums which he has subtly found out to be living creatures."
We're still "standing on the shoulders of giants!"
This animation is very, very cool (but takes a good connection).