266 =F. Galton—Address before the British Association. 
measurements. Most of you are aware of the recent progress 
of what has been termed psycho-physiecs, or the science of sub- 
jecting mental processes to physical measurements and to phys- 
ical laws. I do not now propose to speak of the laws that have 
been deduced, such as that which is known by the name of 
echner, and its numerous offshoots, including the law of 
fatigue, but I will briefly allude to a few instances of measure- 
ment of mental processes, merely to recall them to your mem- 
ory. They will show what I desire to lay stress upon, that the 
very foundations of the differences between the mental quali- 
ties of man and man admit of being gauged by a scale of inches 
and a clock. 
should be lost on the road, in transmitting their impressions to 
the brain. Now the velocity of the complete process of to an 
fro nerve transmission in persons of different temperaments has 
not been yet ascertained with the desired precision. Such dif- 
ference as there may be is obviously a fundamental character- 
istic and one that well deserves careful examination. 1 may 
take this opportunity of suggesting a simple inquiry that would 
throw much light on the degree in which its velocity varies 1n 
different persons, and how far it is correlated with temperament 
and external physical characteristics. Before I describe the in- 
quiry I suggest, and toward which I have already collected a 
few data, it is necessary that I should explain the meaning of 
a term in common use among astronomers, namely, “ rsonal 
equation.” It is a well-known fact that different observers 
make different estimates of the exact moment of the occurrence 
of any event. There is a common astronomical observation, In 
which the moment has to be recorded at which a star that 18 
traveling athwart the field of view of a fixed telescope, cross 
the fine vertical wire by which that field of view is intersected. 
In making this observation it is found that some observers are 
over sanguine and anticipate the event, while others are slug- 
gish and allow the event to pass by before they succeed in not- 
ing it. This is by no means the effect of inexperience oF mal- 
adroitness, but it is a persistent characteristic of each individ- 
ual, however saaiicha: in the art of making observations OF 
