244 
DR. J. YOUNG- AND PROFESSOR G. FORBES ON THE 
discarded altogether when we found that the atmospheric conditions were seldom 
favourable enough to admit of our using a larger aperture than 3 inches. We have 
always used the 3-inch collimators in our observations. Three adjustments must be 
made : (1), for focus ; (2), for centering ; (3), for direction. 
(1.) To adjust the focus, the cap with the mirror attached is removed and another 
one put in its place. This one is similar to it in every way except that the mirror is 
replaced by a piece of ground-glass whose surface has the same position with respect 
to the cap that the mirror has in the other one. The image of a star or the sun can 
be thrown upon the ground-glass and focussed. The number of turns and parts of a 
turn of the screw of the cap are then counted, and the cap with mirror is now screwed 
on to the same position. A final adjustment can be made with the collimators in 
position, altering the focus until an observer at the telescope sees the reflected light 
most distinctly. 
(2.) The adjustment which we call centering is accomplished by the three screws 
which attach the mirror to the cap, and causes the centre of the sphere of which the 
mirror forms a part to be at the centre of the object-glass. To test this point a tube 
1 foot long is put on the collimator projecting in front of the object-glass. At the 
end of this tube a small ring is supported in the centre by three strips of metal. On 
looking through this ring the observer ought to see an image of his eye, and the 
screws are adjusted until this is the case. 
(3.) To direct the collimators so that their axes should point to the observing tele¬ 
scope it was found most convenient to look through the object-glass at the mirror. 
If the head be so far as possible prevented from covering the object-glass an image of 
distant objects is seen in the mirror, and the direction is changed until the light 
coming from the observing telescope is seen in the centre of the mirror. 
The clock .—The clock was constructed by Messrs. E. Dent and Co. specially for this 
research. It is driven by a weight attached to an endless chain passing over the 
drum. On the arbor of the drum, and connected with it by a toothed wheel and 
ratchet, is a wheel of 100 teeth working into a pinion of 10 teeth, which is on the 
same arbor as the scape wheel. This arbor bears a hand which marks seconds, and 
the arbour of the large wheel bears a hand which marks the minutes from 1 to 10. 
Any additional wheelwork was unnecessary for our purpose and would only tend to 
introduce errors. There is a dead-beat escapement, and the pendulum rod is made of 
varnished wood. The bob weighs 6 lbs., and the driving weight only 4 ozs., or one- 
twenty-fourth of the weight of the bob. The clock is best wound up by unhooking 
the weight from the endless chain after having hooked on another higher up. It goes 
without attention for two hours. 
The arbor of the scape wheel bears another similar one, with 120 teeth, for making 
electrical contact once a second, by which means a mark shall be made on the chrono¬ 
graph. The system of electrical contacts adopted by us seemed to be free from the 
defects of those which tend to disturb the pendulum, because the springs whose 
