Photographs of fluid flow revealed with an ultramicroscope

1.Introduction .—When an intensely bright beam of light from an ultramicroscope is passed through moving water, minute impurities which are always present and which cannot be seen in ordinary light, even with a microscope, become visible provided that they are seen against a dark background. These illuminated particles reveal the internal motions of the water without any interference with the flow, such as might be caused if extraneous particles or colouring matter were introduced into the water. The ultramicroscope has recently been used to examine turbulent flow in pipes, and to obtain information on the flow in the wakes behind bluff obstacles. An attempt has now been made to take photographs of some of the views seen when this earlier work was in progress. These photographs are given in the present paper. Photographs of the flow around a long circular cylinder at low Reynolds' number are also given. All the photographs, with the exception of those of fig. 4 (c ) and (d ), Plate 14, are of water direct from the main. It is perhaps of interest to mention that the number of particles (probably mostly minute air bubbles) in the water appeared to differ appreciably from day to day.