100 Mr W. N. Shaw, Some Laboratory Notes. [May 30,. 
wave length of the light can be calculated. The Table gives 
some results obtained in the laboratory for the wave length of 
sodium light. 
The accuracy is of course not of a very high order, but it 
is remarkable that so close an approximation can be got with 
apparatus that is in common use in all laboratories. 
2. An arrangement to measure the length of an object against 
which a scale cannot be lard. 
This method was devised to measure the wave lengths of 
standing waves on the surface of a mercury trough ; it is applicable 
in many other cases. The object to be measured is viewed by a _ 
telescope, and a piece of plane parallel glass is interposed obliquely 
between the object and the telescope, and so adjusted as to reflect 
into the telescope the light from a scale. The scale must be 
placed so that its reflected image in the plane glass coincides with 
the surface of mercury or other object whose length is to be 
measured. The length can be easily read off in this way if proper 
care be taken in adjusting the relative illumination of the scale 
and the surface. An ivory scale with a black background answers 
very well for the purpose. The plate of glass should be thick, as 
two images of the scale are formed, and they will confuse one 
another unless they are separated by a considerable thickness of 
glass. The method is of course similar in principle to the use 
of the Wollaston camera-lucida and to the scale reading of a 
spectrum by reflection from the face of the prism. 
3. A lecture experiment in self-induction. 
It is known that if the poles of an electro-magnet are short 
circuited by an incandescent lamp, the lamp may be made to 
glow for an instant on breaking the battery circuit, although the 
E.M.F. may not be sufficient to make it glow when the current 
through the circuit is steady. This experiment may be made 
more striking by interposing in the battery circuit a revolving 
wheel contact breaker imstead of a key. By means of an open 
wire resistance, interposed between the battery terminal and the 
electro-magnet, the electrode of the lamp can be attached at such 
a point of the battery circuit that the shunt current is just 
not sufficient to make it glow. On rapidly turning the wheel 
contact breaker the energy of the battery is drawn upon to form 
the magnetic field of the magnet, and discharged through the 
lamp, which glows well and steadily in consequence, if the contact 
is good and sufficiently rapidly broken. 
The lamp used in illustration was a 20-volt lamp, and the 
battery referred to, a set of eight storage cells. 
