ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
607 
the direction of its axis and not perpendicular to that direction as in the former case, 
and, similarly, the irregular bright central band will have its general direction along a 
normal section of the tube. So marked is the extension in the breadth of the imaofe 
due to these stripes of phosphorescence (Plate 27, fig. 15) that if the strip of tinfoil 
be a short one its phosphorescent image seems to be long and narrow and to have the 
direction of its length at right angles to the direction of the tinfoil. The same is the 
case when a piece of tinfoil is placed longitudinally upon the tube; the greatest 
elongation of the image seems to be at right angles to that of the tinfoil. It will be 
understood that there is no actual rotation of the image through a right angle; the 
effect is solely due to the fact that the spreading out is at right angles to the direction 
of the length of the tinfoil, and that it is so great as to cause the breadth of the phos¬ 
phorescent patch which it forms to be greater than the length of the strip of tinfoil. 
That the above conclusions as to the law that governs the spreading out of the 
molecular streams are correct was shown by placing strips of tinfoil in oblique 
positions, when it was invariably found that the stripes or striations ran perpen¬ 
dicularly to the direction of the tangent to the strip of tinfoil at the point from which 
they proceeded. In order to test it rigorously, it was determined to place a strip in 
such a curve that the normal planes to the curve would pass through the tangent at 
the corresponding point of the image of the curve, i.e., the curve on the opposite side 
of the tube, each point of which is exactly opposite to its corresponding point on 
the tinfoil. In such a case all the striations must lie along the curve formed by the 
locus of the central patches of phosphorescence, and the result should be a single 
bright curved line of phosphorescence without any spreading out or striated margin 
(Plate 28, fig. 16). The curve in question is evidently a helix, whose pitch is half a 
right angle. On trying the experiment these anticipations were found to be exactly 
fulfilled. It also occurred to us that as the striations are in the normal plane to the 
strip of tinfoil the locus of their consecutive intersections would give the evolute* of 
the curve formed by the strip of tinfoil, and that as such locus it would probably be 
represented by an especially bright line. We tried the experiment with a fine copper 
wire in the form of an ellipse, which was bent round so as to lie on the tube, and it was 
found to answer perfectly. The four-cuspecl shape of the evolute was distinctly marked 
by a bright fine of phosphorescence (Plate 28, fig. 17). 
It is easy to advance from the case of a strip of tinfoil to that of a patch whose 
length and breadth are alike considerable. The molecular streams from beneath the 
elements in the interior of the tinfoil will be hindered from spreading out in any 
direction by the action of the circumjacent elements, and they will therefore be con¬ 
centrated and forced to pass off in a normal direction. The elements at the edge of 
the tinfoil are, however, in a different position, they can still spread out in directions 
normal to the edge of the patch. They are in fact much in the same condition, so far 
* This term is of course not used in its strict sense, for the wire and the phosphorescence are upon 
curved surfaces, and not upon planes. 
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