VELOCITY OE WHITE AND OF COLOURED LIGHT. 
277 
First differential observations for red and blue light. 
After February 11 we never had a clear day or night to continue our research 
until the 21st of the month. We then made observations in the following manner:— 
A thick piece of indiarubber tubing was attached to the top of the pulley which sup¬ 
ports the weights driving the toothed wheel. At its upper end it was attached to a 
string, which, passing over a fixed pulley, was held by the observer. The observer 
adjusted the driving weights so as to be a little in excess of what was required to 
produce equality of the lights. He then fixed the string tightly, and as the weights 
descended, the indiarubber was stretched, and diminished the effective driving weight. 
In this way a beautifully gradual diminution of velocity was obtained, accompanied by 
as beautifully gradual an increase and decrease in the brightness of the two stars 
respectively. Our plan was to place the blue solution between the lamp and the 
diagonal reflector. When equality of lights was attained the observer said “ Stop ” 
to the assistant, who then commenced to count seconds on the clock. At the same 
time the blue solution was replaced by a piece of ruby glass. When equality of red 
lights was attained the observer again said “Stop.” The interval of time was then 
noted in the observing book. Our intention was to measure by means of the chrono¬ 
graph, at our leisure, the diminution in velocity produced by the action of the 
indiarubber during a given number of seconds. 
Alongside of each observation we entered in the observing book the weights we 
used. But in the observations of February 21 these were nearly always the same, 
and it was always the 12th equality which was observed, being a speed of about 400 
revolutions a second. It will nevertheless be well here to tabulate the names of the 
weights employed and their absolute weights, as reference is frequently made in the 
observing book to them. We usually had one large lead weight to begin with, and 
this was the one used when one lead weight is spoken of. It weighed 56 lbs. We 
had two other lead weights which might be added, each weighing 12 lbs. 2 oz. We 
had five iron weights, each weighing 10 lbs. 7 oz. These were generally supported by an 
additional iron hook weighing 1 lb. 11 oz. Smaller weights, two of which were called 
a and b, each weighing about 1 lb., and others weighing a few ounces, were used as weil. 
The length of the indiarubber tubing which we generally, if not always, used was 
10^ inches when unstretched. It lengthens 3 inches with the addition of 1 lb. weight. 
With one lead weight (56 lbs.) the fall of the weight when driving the mechanism is 
1 inch in 7 seconds. 
With three lead weights, one iron weight and hook (92 lbs. 7 oz.), the fall is 1 inch 
in 5*3 seconds. 
[N.B .—It must be remembered that the effective weight is half of the actual weight 
owing to the action of the pulley.] 
On February 21 eight observations were made in the manner described above. A 
little practice was required to get accustomed to the method of observation, but after 
