VELOCITY OF WHITE AND OF COLOURED LIGHT. 
249 
observation so as to send a maximum quantity of light along the axis to the distant 
reflectors. In some of our later experiments we used a bisulphide of carbon bottle- 
prism to send particular colours to the distant reflector. This prism was laid upon a 
ledge on the support of the condensing lens and immediately below it. With a little 
care a very pure spectrum could thus be formed. 
Establishment of the apparatus at Kelly , Wemyss Bay. 
The room in Kelly House (the property of Dr. Young) which was used for the 
observatory was the billiard room, facing the front of the house (the west). It is the 
room immediately to the north of the entrance hall or lobby. A pane of glass was 
removed from the window for observation. On the window-sill a brass plate has been 
fixed, with an inscription, indicating the exact relative position of the toothed wheel. 
The telescope was mounted upon two strong wooden supports. The toothed wheel 
mechanism rested upon two very solid beams (between which the cord for the weights 
could pass). These beams rested at one end upon a standard fixed to the wall of the 
room, and at the other end upon a strong box which rested on the billiard table. This 
end was also supported by a standard; and the floor was strengthened by thick 
wooden props below. The horizontal beams also acted as a support for the framework 
bearing the condensing lens and prism. The electric lamp was on a separate table. 
At one end of the beams and at the observer’s right hand mercury cups were placed, 
with metallic contact pieces, by means of which the batteries could be put out of 
action when not required. Leclanche cells were used for the chronograph connexions. 
Twelve cells were used in all. The clock was attached to the wall of the room facing 
the observer. The levelling screws of the chronograph rested upon three metal plates 
on a piece of wood on the billiard table. 
We have inserted a brass plate in the stone outside the window, with an arrow 
pointing towards the position occupied by the toothed wheel, the distance from the 
point being 106 inches. 
Establishment of the reflecting collimators. —The reflecting collimators are placed on 
the hills behind the village of Innellan, separated from the observing telescope by 
the Firth of Clyde. The nearest one (B) is on the summit of the hill called the Tom* 
It rests upon two iron Y’s which are imbedded in the solid rock. The reflector was 
8 inches further from Kelly ( i.e ., more to the west) than the most westerly of these 
two Y’s, whose positions will always be marked by the holes. The collimator was 
covered by a wooden box with a hole in the east side. This box was attached to four 
iron rods placed in holes in the rock. 
The more distant reflector (A) is in the face of the hill to the west of the Tom, 
nearly in the same line as a line drawn from Kelly to the top of the Tom, but two 
feet to the north of this line. It was impossible to gain a solid foundation, but the 
* Kelly House and the Tom are both shown on the Ordnance Survey Maps. 
MDCCCLXXXIL 2 K 
