The early fluidic and optical physics of cytometry

All forms of cytometry, depend on the basic laws of physics, including thos e of fluidics, optics, and electronics, most of which were established cent uries ago. Flow cytometry depends critically on the fluidics presenting eac h individual cell with precision to the sensing volume. This is intersected by a high-intensity light source, and light scattering and fluorescence fr om suitably stained constituents in each cell are captured by the light-col lecting optics and measured. The works and observations of Bernoulli and Eu ler in the 18th century, Reynolds in the 19th century, and Crosland-Taylor in the 20th century in the field of fluid dynamics laid the foundations for hydrodynamic focussing, which is the primary prerequisite for presenting i ndividual cells to the sensing volume. In addition, electrostatic cell sort ers must have the ability to generate stable droplet formation in the jet-s tream issuing from the flow chamber nozzle. The origins here can he traced to work carried out in the early to mid-19th century by Savart, Magnus, and Thomson. Flow, image, and confocal cytometry are all dependent an the laws of optics , including those of reflection and refraction as well as numerous other op tical principles. The observations and works of Socrates, Ptolemy, Snel, an d Descartes between about sc 370 and 1637 were of seminal importance in dev eloping the laws of reflection and refraction. In the mid-17th century Hook e illustrated the power of magnifying glasses and microscopy in his Microgr aphia and Newton was responsible for explaining colours in the spectrum. Hu ygens, toward the end of the 17th century, put forward the concept of point source light propagation contributing to a wave front. Finally, Thomas You ng, early in the 19th century, established the wave form of light from inte rference patterns. Most people will be familiar with some of these discoveries and the investi gators who carried out the work; some people will be familiar with all of t hese. However, very few people are likely to have had the opportunity and p rivilege to access the very early works and the original data and manuscrip ts, or translations thereof, which laid the foundations of physics that ena bled our discipline to be established. It is always important for any disci pline to remember its roots and to appreciate the seed from which those roo ts grew, for it is much easier to learn and fully understand when we have a knowledge of the source and the logical progressions that lead from one di scovery to the next. This knowledge lends perspective to our current endeav ours as the past, after all, created the present, which in turn contributes to the future. In this article, which was presented as an invited lecture at the 9th Canad ian Consensus meeting on AIDS, I have attempted to trace the origins of the early work an the physics of fluidics and optics, which laid the foundatio ns. Cytometry (Comm. Clin. Cytometry) 38:2-14, 1999. (C) 1999 Wiley-Liss, I nc.