112 MODES OF MEASURING MINUTE INTERVALS OF TIME. 
Art has more control over the accessary movements by which the phenomenon is re- 
ferred to a known instant. In this class of incidents, either the same organs may be em- 
ployed throughout the whole observation, or different organs may be called into exercise. 
The employment of one sense, or of the eye alone, has not been greatly practised, nor 
have the results been satisfactory. It was proposed by Breguet of Paris, about thirty years 
ago, to introduce a time-movement into the field of view of the telescope, to be studied 
simultaneously with following the course of the star. To this, Brewster objected, that 
when attention was concentrated on one object, the other would disappear from view. 
This would generally be the case, and to a person trained to other modes of observing, the 
difficulty might be perplexing. But if the habit had been acquired of watching the con- 
tact, and then instantly by an immediately subsequent effort, catching the indication of 
time, this would have required an interval, as all such operations must do, but all that 
would be needful for accuracy is, that personal training should render the interval con- 
stant, and that it be allowed for. It might in making an attempt to observe in this way, 
be found suitable that the time-movement were brought into the field of view by reflec- 
tion only. 
The more common modes of observation, call for the exercise of more than one organ. 
Of these modes the older employed seeing and hearing; the practice being to count se- 
conds as beat on a clock, up to that immediately preceding the expected contact, and then 
to estimate the position in time of the phenomenon of contact between the beat last heard 
and the succeeding one. The estimate in this case might have been rendered dependent 
on the habit, reduced to regularity by training, of repeating in thought the numbers up to 
eight or to ten; and various modes of giving accuracy to the estimate might have been 
introduced. But the whole process will probably be everywhere superseded by the mode 
introduced and recommended by our American astronomers, by which these accessary 
movements are considerably simplified. 
Tn this case we employ the eye to watch the phenomenon, and some other organ, such 
as the finger, to give a signal; the signal being a permanent mark by electric influence on 
some apparatus for time-movement, so that the instant of apprehended contact is regis- 
tered on the apparatus. "This improvement relieves the observer from all necessity of at- 
tending in thought to intervals of time. It leaves him free to watch for the single phe- 
nomenon of contact. Himself and the earth’s rotation are now the only elements con- 
cerned in the proceeding. In. the case of there being several parallel wires in the field of 
view, no process need interfere to direct his attention from the successive contacts. 
If we bring under consideration now the condition of the observer, we shall find two 
circumstances remaining which will still give origin to differences of result in different in- 
dividuals, constituting personal equations to be investigated and taken into account in re- 
