248 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
wires in its focus. This microscope can be raised or lowered on the pillar which 
supports it. On looking through this microscope the marks made by one of the pens 
can be seen and brought, by rotating the fly-wheel, into super-position on the cross 
wires. The position of the cylinder is then read off by the vernier. The circle is 
divided into 500 equal parts, and by aid of the vernier we can read to one-twentieth 
part of one of these divisions, i.e., to too 00 ^ of a revolution. We found it best in 
practice to rotate the apparatus at the rate of about one revolution a second. Hence 
one division of the vernier corresponds to about xotrooth of a second. 
The perfect working of the chronograph depends upon a number of conditions which 
must all be fulfilled simultaneously. They are dependent (1) upon the manufacture 
of the instrument, (2) upon the mechanical adjustments, and (3) upon the electrical 
adjustments. The two last alone can be improved by the observer. The principal 
objects to be sought after in the manufacture of the instrument are absence of friction, 
rigidity, and the reduction of the moment of the momentum of the fly-wheel and 
cylinder together about their axis to zero.* The mechanical adjustments are as 
follows :—The vertical position of the axis of the instrument must be carefully tested 
by a level. The points of support must be oiled. The screws connecting the cylinder 
to the fly-wheel must be quite tight. The pressure of the upper point of support 
upon the axis must be very nicely adjusted. This point of support must be kept 
rigidly in position by the nut. The electrical adjustments are as follows :—The 
batteries and magnets must be in good working order. The pens must move freely 
and over a suitable range, and they must press with a suitable pressure upon the 
smoked cylinder. 
The dynamo-electric machine. —The electric current to work our electric lamp 
was obtained from a small sized Siemens’ dynamo-electric machine requiring three 
horse-power and rotating at the rate of 1,400 turns a minute. The axis of the 
machine is attached directly to a turbine of that kind known by the name of a vortex 
turbine. This was fed by a supply of water with a head of 300 feet coming from a 
distance of about a mile, and led to the house by a 3-inch water pipe. The current 
was led by means of stout wires from the generator to the lamp, a distance of about 
100 feet. No further account of this machine is necessary, as our practice only, and 
not our results, could be affected by any imperfections. 
The source of light .—We employed for our source of light, in the experiments of 
1880-81, a Siemens’ electric lamp. A condensing lens was used to throw an image 
of the incandescent carbon, after reflection by the diagonal reflector, upon the toothed 
wheel. Since with this lamp motion is given to only one of the carbon points it was 
necessary to mount the condensing lens upon a framework which should admit of its 
having an up and down movement. For the same reason the diagonal reflector in the 
eye-piece of the telescope was turned round about the axis of the telescope at each 
* The maker did not quite fully carry out our ideas on the two last of these points. The rigidity of 
the apparatus would have been increased by having three vertical pillars in place of two. 
