224 



OPTICAL PROJECTION 



A pretty method of exhibiting capillary attraction has been 

 described by Mr. G. M. Hopkins. A piece of wire-gauze with 

 the wires about one-tenth of an inch apart is thinly varnished, 

 and projected as a lantern slide, in any way that leaves the 

 whole front of the gauze accessible. Then a few differently 

 coloured fluids of almost any kind aniline dyes answer very 

 well being provided in small saucers, with a clean hair pencil 



for each, a drop of fluid is intro- 

 duced by the point of the brush 

 into .each separate mesh. This 

 is easily done so as to exhibit a 

 geometrical pattern, which when 

 magnified has a wonderfully fine 

 effect upon the screen. By 

 * drawing ' the brush over the 

 meshes the effect is different, 

 and will resemble that of Berlin- 

 wool-work upon a large scale, 

 the colour always running out to 

 the edges of exact squares. 



Experiments with fluids in 

 tubes project better if the tubes 

 can be somewhat flattened whilst 

 red-hot. The flat sides are of 

 course turned towards condenser 

 and screen, the object being to 

 get more surface and a nearer 

 approach to one focal plane. 



Endosmose is readily projected from the usual apparatus 

 shown in fig. 113, using a nearly capillary tube, which is pro- 

 jected, and rather large membranous bag, and staining the 

 internal fluid. If the tube is sufficiently small in propor- 

 tion, the rise of the fluid in it will readily be seen in pro- 

 gress. 



118, Surface Tension, The tension of liquid surfaces isi 



FIG. 113 



