SPANGLED TUBE. 603 



the luminous spiral appears as if a fringe of delicate 

 luminous filaments were attached to it. 



A tube of suitable length and width, being selected, the sharp 

 edges at the ends are rounded off over the flame. When the hot 

 ends have become cool again, the whole tube is moderately warmed 

 and dried inside by drawing air through it. Both ends are then 

 closed by tight-fitting corks, which are cut flush with the ends of the 

 tube. A square piece of tinfoil is pasted round one end of the tube, 

 a length of l cm being left to project beyond the end ; this margin 

 is afterwards folded over so as to cover the cork. Moderately 

 thick starch paste is used, and the tinfoil covered on one side with 

 a uniform thin layer of it. Only one edge of the strip of tinfoil is 

 placed upon the tube at first, and pressed upon it by drawing the 

 finger or a small plug of cotton wool along it ; the remainder of the 

 tinfoil is then gradually laid round the tube, rubbing constantly so 

 as to make it firm and smooth. It is desirable to leave only a trace 

 of paste between the glass and the tinfoil, yet the pressure applied 

 should not be too great, or otherwise those portions which are 

 already attached may be again displaced. The folds in the portion 

 over the cork should be pressed smooth with a finger-nail, and a 

 small disc of tinfoil, 3 or 4 mm narrower than the tube, may be after- 

 wards pasted upon the end. 



Several long strips of tinfoil 3 or 4 mm wide are cut with a sharp 

 knife, using a ruler, and placing the tinfoil upon a sheet of zinc; the 

 strips are pasted spirally round the tube, nsing isinglass for the 

 purpose instead of starch. The tube must then immediately be 

 carefully wiped quite clean with a damp cloth and left a day for the 

 strips to become firmly attached. By means of double cross-cuts 

 the strips are divided into a number of small hexagons, and the 

 intervals between them cleaned from the adhering bits with a needle 

 or the point of a knife. The mode of making the cross-cuts is 

 indicated in fig. 318, B. The distance between two adjoining 

 hexagons should be from 0'5 to l mm . In cutting, the sharp edge of 

 the knife, not the point, must be used, otherwise the tinfoil will be 

 torn from the glass in many places ; no great pressure should be 

 applied, as the glass soon blunts the knife without it ; the hone 

 should therefore frequently be applied. 



The knob is formed of a brass ball with a short stalk which may 

 easily be obtained from a dealer in electrical apparatus ; the stalk is 

 fixed in the cork, a hole of suitable size being made in the latter with 

 a cork-borer. Care must be taken that the ball is in metallic com- 

 munication with the end of the spiral near it. Brass knobs suitable 



