With Observations and Inquiries Thereupon” small structural units that decided to name “cells” or “cellulae”. Robert Hooke (1635-1703) discovered and described in his significant book entitled “Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. Description of events taking place in biological systems was always one of the main targets of the optical microscope, despite the limited spatial resolution. This means that in the visible region of the electromagnetic spectrum one can appreciate, for example, the adhesion of a biological cell to a surface without reaching the ability to see the cellular protein distribution at the molecular scale, Fig. As a consequence of this physical constraint, kindred objects that are together at a distance closer than 200 nm cannot be discriminated and “minutiae” of the way they are distributed in a certain space can be seen in an image only to a certain degree of visibility. In practice, considering the phenomenon of diffraction, light cannot be focused to an infinitely small spot, which for visible light, considering transparent objects, like most of the biological molecules, using a glass lens in air, n = 1, water, n = 1.33, or immersion oil, n = 1.52, amounts to 200–250 nm. In terms of frequency, this compares to a band in the vicinity of 430–770 THz. The most used simplification is given by considering light in the visible region that for the human eye corresponds to wavelengths from about 380 to 740 nm. This simplified formulation is now enough for our purposes and for the image formation properties we are interested in. At first glance, the spatial resolution, d, referred to the minimum distance between two objects to be recognized as separate entities in the x–y image plane is approximatively given by: Ernst Abbe coined the term “numerical aperture” for the quantity NA = nsin \(\alpha \). Lenses are often manufactured from specifically composed glass featuring well defined characteristics as function of the shape resulting in the semiangular aperture, \(\alpha \), of the wavelength of the light being used and the refractive index of the not absorbing propagation medium (n), Fig. This is the reason why the overall performances of the optical microscope are generally referred to the ones of the objective utilized to form images. In fact, lenses are arguably the most fundamental components of any microscope. The lens is the key element of the Abbe’s experiment. As recently argued by Sheppard “the use of the word ”limit” implies a definite and sudden transition from being resolved to not being resolved, in consideration of the original sentence reported by Abbe “Die physikalische Unterscheidungsgrenze dagegen hängt allein vom Oeffnungswinkel ab und ist dem Sinus seines halben Betrages proportional.” translated as “The physical limit of resolution, on the other hand, depends wholly on angular aperture, and is proportional to the sine of half the angular aperture.”. The most popular relationship to address the spatial resolution performances of an optical microscope in the observation plane (x–y) is the so-called Abbe’s law that represents a heavy stone on the shoulders of any microscopist, Fig. The first “station” is the one related to the quantitative and fundamental turning point in optical microscopy that occurred, in 1873, when Ernst Abbe (1840–1905) studied the physics of lens construction and its relationship with the performances of the lens in terms of spatial resolution. It is worth mentioning three of them without loss of relevance for all the others.
It is a matter of fact that the road to nanoscopy passes through many stations.
We can rephrase the Faber’s sentence into “Nanoscopium nominare libuit” in a way that optical microscopy turns into optical nanoscopy. Today the optical microscope permits to design and perform experiments at the nanoscale and beyond since the diffraction barrier is crumbling. “Microscopium nominare libuit” referred to Galileo Galilei’s (“small eyeglass” (occhialino) is clear about the ability of such an instrument to provide a direct view of details before invisible to naked eyes by using a properly shaped piece of glass and the sunlight, the colours of the rainbow. Johannes Faber, a fellow of the Accademia dei Lincei, in 1625, in a letter to Federico Cesi, conceived the term microscope writing a sentence that makes clear how the advent of the optical microscope was a turning point in science. The light microscope provided scientists with a kind of extension of the human eye endowed of the capability of observing details below 0.1 mm offered by human eye physiology and anatomy.
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