314 
PROFESSOR HELE-SHAW AND MR, ALFRED HAY 
to be projected on the screen, it allowed, when illuminated from behind, sufficient 
light to pass to enable the stream-lines to be photographed. At first the difficulties 
of using paraffin wax were numerous, for on casting the wax and allowing it to cool, 
it became covered with star-like cracks beneath the surface, which were evidently 
caused by unequal heating and cooling, and imprisoned air bubbles. Casting in a 
vacuum was also tried, but without success. The difficulty was at length overcome 
by a systematic method of covering the plates, which was as follows : The plate on 
which the layer of wax was required, was placed in an iron tray, the wax was 
shredded on to the plate, and the whole heated gradually and uniformly, a plate of 
iron 1 inch thick being inserted between the tray and the Bunsen burner used for 
heating. By this means the plate could be covered to any required thickness in a 
satisfactory manner. In order to reduce the layer of wax to the thickness required 
for the experiment, the simple planing machine illustrated in figs. 7 and 8, was used : 
Fig. 8. 
Plan. 
Planing Instrument for Paraffin Surface. 
the gauges (AA) for obtaining the required thickness being shown in position. The 
milled-headed screws enabled the cutter to be adjusted so that the desired thickness 
of v ^ax could be accurately obtained of any dimensions above '004 inch to within '001 
inch with the greatest ease. The method of illuminating the slide had, however, to be 
abandoned, as the heat of the arc lamp melted the wax, thus at once putting an end 
