Tis not unlikely, but that there may be yet invented several other helps for the eye, at much exceeding those already found, as those do the bare eye, such as by which we may perhaps be able to discover living Creatures in the Moon, or other Planets, the figures of the compounding Particles of matter, and the particular Schematisms and Textures of Bodies.
– Robert Hooke, Micrographia (1665)
A well-flogged cliché is the image of a scientist peering into a microscope. Unforgivably numerous B-movies have depicted some delusional maniac, preferably with wild hair and a white lab coat, staring at an impossible entity of doom (purple-fluorescent virus anyone?) through the lens of a compound microscope. Yet, the very existence of this stereotype is because the microscope has revolutionised science, particularly biology. While irreversibly associated with biologists, modern microscopy brings together physics (optics, and now electronics) with chemistry, to enable modern biology. The story of the light microscope is also the story of the telescope. Indeed, our ideas about the outermost expanses of the universe and the innermost workings of a cell come from our ability to see using lenses and push the boundaries of what physics and chemistry will allow.
As with most stories, this one encompasses incredible personalities who catapulted the field forward. Perhaps the greatest of these was Robert Hooke. Middle school children will know of Hooke only for the most mundane of discoveries, Hooke’s law of elasticity. Yet Hooke was a colossus of science, contributing to optics, microscopy, planetary motion and gravity, and helping rebuild London after the great fire of 1666 nearly destroyed that city. His is a story of greatness, where petty jealousies resulted in him not being fully remembered for his mammoth contributions to science.
Unlike most wealthy “gentleman” scientists of that time, Hooke was self-made. He was born to a poor curate who died when he was young, became a working apprentice and embarked on a lifelong study of mechanics. And due to his abilities in mechanics, he worked for Robert Boyle (of Boyle’s law fame), who recognised his abilities and became a lifelong supporter. With Boyle’s support, Hooke got a job as the Curator of Experiments at the Royal Society (then still an amateur organisation). This was a tough job; Hooke had to demonstrate other people’s results when reported to the Royal Society using his own experiments. He was fascinated by optics, and explored the use of simple and compound microscopes to observe “small things”.
Hooke v. Newton
His contemporary, a Dutch merchant called Antonie van Leeuwenhoek, was similarly obsessed with lenses, and while peering through his simple microscopes observed small animalcules in liquids, the first observation of microbes. Leeuwenhoek communicated this to the Royal Society, and it was largely due to Hooke’s efforts (as Curator of Experiments) that Leeuwenhoek was recognised for his discoveries. Independently, Hooke made tremendous observations using his relatively primitive compound microscope, which resulted in his great written endeavour, Micrographia. This was one of the first scientific books written in English (and not Latin). His beautiful, detailed diagrams of organisms and artifacts, and simple writing style made this the first ever scientific bestseller. Importantly, Hooke observed hexagonal structures in a layer of cork, which he called “cells”. Thus, the name for the cells that make up living beings first came into use and remains so to this day.
Sadly, Hooke is remembered poorly due to being disliked by his contemporary, Isaac Newton. Newton, for all his genius, was possessive and vindictive. It didn’t help that Hooke was prickly (conscious of both his humble origins and his severe physical deformities) and argumentative. The ultimate polymath, Hooke was also interested in the behaviour of planetary bodies and in communications to the Royal society in the 1670s, he postulated:
i) That all the heavenly bodies have not only a gravitation of their parts to their own proper centre, but that they also mutually attract each other within their spheres of action. ii) That all bodies having a simple motion, will continue to move in a straight line, unless continually deflected from it by some extraneous force, causing them to describe a circle, an ellipse, or some other curve…”
As we all know, the second statement largely describes Newton’s second law. Newton had already been working on the attraction of planetary bodies for a long time, but was a decade away from publishing Principia. While Hooke was perhaps not as capable a mathematician as Newton, he did make these observations independently and came close to revising these statements and outlining an inverse square law of gravity. Hooke’s rivals belittled him to Newton, who ended up suspecting Hooke of undermining his efforts and claiming credit for Newton’s work, as well as being irritated by Hooke’s argumentative personality.
After Hooke’s death, Newton became president of the Royal society, and with exceptional vindictiveness expunged every memory of Hooke from the Royal Society’s records (including destroying the only portrait of Hooke). Many of Hooke’s greatest contributions were reduced or attributed to someone else, including the optical phenomenon of “Newton’s rings”, first described in Hooke’s Micrographia. Newton mocked Hooke in his famous quote: “If I have seen further, it is by standing on the shoulders of Giants”, where Giants was capitalised to mock Hooke’s deformed body and suggest that Hooke’s contribution was insignificant compared to other scientists (particularly Descartes) whose work enabled Newton’s research.
Surmounting Abbe’s law
Back to the microscope. Hooke’s Micrographia (along with Leeuwenhoek’s observations) helped create a surge in interest to improve lenses and the compound microscope. This led to an explosion of knowledge, including the discovery of microbes, living organisms invisible to the eye. The first lenses only allowed a detailed observation of small insects (“flea lenses”) but the compound microscope, by using multiple lenses in combination, allowed an exponential increase in magnification. For example, a single lens that could magnify something four-times when combined with a second lens could magnify the same object 16-times.
Great advances were made in microscopy starting in the late 19th century, particularly at Zeiss (still a leader in microscopy), which has now led to the observation of small mammalian cells, bacteria, and even small “organelles” within a cell, such as the mitochondria. Yet, there was a stumbling block, calculated by the German scientist Ernst Abbe (who co-founded Zeiss microscopes) in the 1870s.
Abbe brought precise mathematics and physics to the development of microscopes, which till then was mostly trial and error, and eventually postulated that the maximum resolution of a microscope would be limited by the wavelength of light. This Abbe’s limit, d=λ/(2n sinθ), says that the resolvable limit ‘d’ depends upon the wavelength of light, which meant that conventional light microscopy could not allow the visualisation of anything smaller than half the wavelength of light, which is 200 nm. Yet, a lot of biology is smaller than 200 nm (viruses for example), as are most individual molecules within a cell. To fully understand a cell, we need to understand how these molecules work within a cell, and overcoming the Abbe limit therefore became one of the great feats of modern science.
This was finally achieved through the development of super-resolution microscopy. To do this, two types of systems were created. In one, called stimulated emission depletion (STED), two laser beams are used, one to stimulate a fluorescent molecule to glow, while the other cancels out the fluorescence of everything around it except at a nanometer-distance around the molecule. Separately, another method allowed scientists to turn on and off the fluorescence of a single molecule, and this became single molecule spectroscopy.
Both these methods were disciplines that combined chemistry and physics at the deepest level. Without the invention of a range of fluorescent molecules, as well as lasers and optics, these two methods would never have emerged. Together these methods allow biologists to finally see what single molecules do in a cell, where exactly they are and how they behave. It was not surprising that the discoverers of these methods (Hell, Betzig and Moerner) received a Nobel Prize in 2014. The only question was which discipline this would win a Nobel for – since it relied on advances in physics and the development of unique chemical molecules to tell us what happens to molecules inside a cell (a quintessential biological question). The Nobel Prize was given for chemistry.
In a way, all this can be traced back to the likes of Robert Hooke, who with their insatiable curiosity for science and love for physics, chemistry and the natural world started off this revolution in microscopy, and the discovery of the very small.
Sunil Laxman is a scientist at the Institute for Stem Cell Biology and Regenerative Medicine, where his research group studies how cells function, and how they communicate with each other. He has a keen interest in the history and process of science, and how science influences society.
Note: This article was updated to state that half the wavelength of light measured 200 nm, not 2 nm as was stated earlier.