472 Notices respecting New Books. 



giving as example the image represented in fig. 34 b, given at 

 high exhaustion on the wall of a spherical vessel by a regular 

 pentagon of 12 millim. in the side bent out of wire about 

 1 millim. thick (fig. 34 a). The normal to the polygon at its 

 centre point was placed radially. 



The waving contour-line which passes through the ends of 

 the five-rayed star forms a perfectly sharply defined contour- 

 line. 



If we employ a regular wire polygon of some other number 

 of sides (3-8), we obtain a similar luminous star with the 

 corresponding number of rays. With polygons with an odd 

 number of sides, the rays of the star correspond in direction 

 to the longer radii of the kathode-polygon. 



With polygons of an even number of sides, on the other 

 hand, the axes of the rays correspond to the shorter radii of 

 the polygon, and thus appear to intersect the sides of the 

 polygon at right angles. 



With the phenomena which are given by interrupted 

 kathodes may be connected an observation made with plates 

 perforated with holes, which is at first sight surprising. If 

 we make use of a square plate perforated with a number of 

 holes (fig. 35), we might perhaps expect that the places from 

 which the metal has been removed Avould appear dark in the 

 phosphorescent images, or at least would correspond to 

 mimima of light. We find, however, that these points are 

 really maxima of light ; thus with the kathode fig. 35 we 

 obtain 16 very bright points of light. The reason is to be 

 found in the fact that the walls of each perforation form a 

 short open tube ; and, according to what we have seen, a bright 

 pencil of light issues from such a tube, which at suitable 

 exhaustion extends to the wall and excites phosphorescence 

 there. 



Berlin, Physical Institute 

 of the University. 



LII. Notices respecting New Books. 



Graphical Determination of Forces in Ehigineering Structures. By 

 James B. Chalmees, G.E. London: Macmillan. 1881. 405 + 

 xxvi pages, 6 plates, '267 cuts. 

 HPHLS is a large and important work, aiming at being a complete 

 ■*- treatise on use of graphic methods in engineering-designs. It 

 is a high-class work, requiring a fair knowledge of modern geo- 

 metry for its comprehension. To facilitate this (to the Engineer) 

 a special Chapter on ' ; Projective Geometry " (06 pages) is given ; 

 within this compass a wide range is compressed, e. g. projections, 

 homology, Carnot's, Pascal's, Desargues's theorems, &c. 



