VELOCITY OP WHITE AND OP COLOURED LIGHT. 243 
which gives it motion, considerable forethought is necessary. It must revolve at 
a very great speed, and must be capable of going at least for some minutes 
so as to avoid the necessity of continually winding it up. Our apparatus was con¬ 
structed by Messrs. E. Dent and Co. It is mounted upon, three screws, which rest in 
three holes countersunk in a plate of iron, which latter has an oblong hole in it to 
admit the passage of the catgut supporting the weight. This base plate of iron is, 
as before stated, generally slightly inclined by resting on a wedge, so that the light 
striking the toothed wheel is reflected upwards. Each wheel works into a pinion 
so as to multiply the velocity tenfold. These wheels and pinions are on five separate 
arbors, so that the multiplication is altogether 10,000-fold. On the first arbor the 
drum, 3 inches in diameter, is fixed. To the second a circular disc with a milled edge 
is attached. This enables the observer to increase or diminish the velocity of the 
wheel by a touch of the hand. It also enables him to reduce the velocity before 
putting on the brake, a sudden application of which might injure the mechanism. 
On the third arbor there is a cam which makes contact between two platinum points 
supported by two springs once in a revolution (i.e., 100 revolutions of the toothed 
wheel), by means of which a current of electricity is transmitted to the chronographs. 
On the fourth arbor there is a brake consisting of a light disc of metal caught between 
two springs. To release this brake a handle or key behind the mechanism is turned 
through a right angle. This separates the springs and the brake ceases to act. The 
fifth arbor supports the toothed wheel, which is 1-g- inch in diameter, and contains 
400 teeth cut to a depth of y^-th of an inch. The wheel is bevelled so as to reflect 
the light away from the observer’s eye. Teeth of different shapes were tried, but the 
best was found to be that of saw teeth (i.e., with pointed teeth and pointed spaces). 
The width of the teeth could then be varied by raising or lowering the mechanism 
by means of the foot screws. In the same way the toothed wheel could be brought 
exactly into the focus of the telescope. Wheels with varying numbers of teeth 
were also tried, and that one with 400 teeth was chosen as giving the best results. 
The reflectors .—The two reflecting collimators are of identical construction. Those 
which we found to be most suitable were a pair constructed by Messrs. Tboughton 
and Simms. They are supplied with achromatic object-glasses of 3 inches diameter 
and 3 feet focus. At the other end of the tube a cap is screwed on, and to the centre 
of this cap a circular silver mirror is attached by three screws which admit of adjust¬ 
ment. This mirror is ground to a spherical form, the radius of the sphere being 
3 feet, and the centre of the sphere of which it is a paid being the centre of the 
object glass. Other collimators were tried, in which the object-glasses were replaced 
by 9-inch silvered glass reflectors of parabolic form. This pair was constructed by a 
maker who makes a speciality of such reflectors for telescopes; but the whole work¬ 
manship was so disgracefully bad in every part, every conceivable fault being found 
in it, and every known device for patching up and concealing bad work having been 
resorted to, that in the form supplied to us they were absolutely useless. They were 
2 I 2 
