372 Br J Ophthalmol 2002;86:372 Coverillustration......................................................................... Br J Ophthalmol: first published as 10.1136/bjo.86.4.372 on 1 April 2002. Downloaded from The eyes have it... hristian Huygens (1629–95) the during the Ordovician period to survive internal reflection and would have lim- Dutch mathematician, and Rene until the end of the Devonian period. ited light collecting efficiency. CDescartes, the French These ommatidia were arranged in a Thanks to Levi-Setti et al (The Eye: Palae- philosopher/mathematician who first hexagonal packing pattern but were ontology; Frontiers of Life. London: Academic “correctly” explained the rainbow found in a rectilinear pattern with verti- Press, 2002) the evolution of the optics of (1596–1650), independently solved the cal rows, as can be seen from the two these creatures is becoming calcite clear. problem of spherical aberration in a lens, examples of trilobites, Phacops rana mill- These investigators suggest that the first with a doublet lens arrangement; a eri and Phacops rana crassituberculata, trilobite probably had holochroal eyes system that now bears their names. shown on this month’s cover. These indi- with only a single calcite lens in the cone Unbeknown to them, though, evolution vidual ommatidia were generally larger of the ommatidium, although there had already solved this problem millions and had an array different from the would be optical difficulties with these of years earlier in small ancient creatures holochroal eye. The ommatidia were of eyes including diffraction limitations. known as trilobites. This subtle, recently different sizes, becoming larger towards With room for improvement, the evolu- decoded, palaeontological tale reveals the ventral surface of the animal. The tionary tinkering that continued would the microevolutionary principles con- first lens in each ommatidium was prove advantageous—especially in a tinuously at work and capable of provid- spherical and was followed by an odd mesopelagic environment—to those spe- ing ingenious solutions to the physical bow-shaped lens with a wave-shaped cies which could evolve improved optics. problems of vision. surface facing the first lens. These two A second step would be the appear- Trilobites arose from the evolutionary lenses are defined in the optical world as ance of an abathochroal eye with a larger crucible of the Cambrian explosion 535 round lens as these animals extended million years ago and evolved for per- into a dimmer environment. As the lens haps 300 million years, only to become enlarged the optical system would be less extinct by the mother of all dyings—the limited by diffraction but rather would Permian extinction, 235 million years be limited by spherical aberration pro- ago. Nevertheless, much is known of ducing a blurred focus. these animals from the fossil record The third, and truly revolutionary, step because these extinct marine arthropods to be taken was the formation of a lens had hard body parts, which included cal- doublet with a perfected Huygensian cite lenses and allowed preservation in profile. This dramatic but logical evolu- stone. Much remains in the fossil record tionary step would allow for dramatic to assist our understanding of the visual increased light gathering capabilities http://bjo.bmj.com/ capabilities and experiences of these and would eliminate spherical aberra- oldest eyes. a “doublet.” It is this second lens, or the tion, as Descartes and Huygens would From this fossil record, we know that Descartes-Huygens correction lens, that learn in the 1600s. Chromatic aberration there were at least two different morpho- is unique, and perhaps seen in only one would probably still exist in such a dou- logical types of these compound eyes, or two other animals, and provides for blet but in a monochromatic world of including holochroal and schizochroal. A the correction of spherical aberration. mesopelagic light or perhaps even epipe- third somewhat intermediary type, This lens system would have increased lagic light—this would matter little. on October 2, 2021 by guest. Protected copyright. known as the abathochroal eye, is some- the contrast sensitivity by as much as Trilobites, especially because of their what less certain, but should probably be fivefold and, in effect, lowered the hard body parts, have contributed much considered as a separate morphological f-number of the eye (see the February to our understanding of the evolution of form. 2001 BJO cover essay for a discussion of the eye and to evolution in general, even The holochroal eye is the earliest and the f-number). Because of the convexity though these hardy pioneers were lost to most common model and consists of of the first lens surface, these schizo- the Permian extinction. To overcome small, multiple, closely packed, round or chroal eyes could even have had stereop- spherical aberration, these creatures hexagonal ommatidia. These individual sis from adjacent ommatidia in the same didn’t need Descartes or Huygens to help visual units are covered by a single eye, although it is doubtful the neuro- them understand the principle of the continuous blanket layer of calcite that is logical mechanisms for this were doublet lens. Long before the appearance in essence the “cornea.” This visual present. of these two scientists, the eyes had it. surface could have been shed with ecdy- These trilobite species had unusual sis (moulting), allowing for growth of abathochroal lenses with the central Ivan R Schwab, the organism as well as for the eye. Iden- nipple appearance creating an aspherical University of California, Davis, Department of Ophthalmology, 4860 “Y” St Suite 2400, tical new ommatidia were added below proximal surface that would help with Sacramento, CA 95817, USA; the existing ones in a hexagonal pattern, spherical aberration and would create [email protected] much like a honeycomb. lenses with bifocal capabilities––a feat Photographs generously loaned by Riccardo The schizochroal eye, a unique varia- emulated by contemporary intraocular Levi-Setti, PhD, from his book Trilobites, tion in compound eyes, was found only lens design. Most likely, though, this sys- Chicago: The University of Chicago Press, in the suborder Phacopina, and was seen tem would have created problems with 1993. www.bjophthalmol.com.