142 ANIMAL PHYSIOLOGY. 
practice. The value of the results obviously depends on the 
reliability of the observer in two respects: First, as to the ac- 
curacy, extent, and delicacy of his perceptions; and, secondly, 
on the inferences based on these sense-observations. Much 
must depend on practice—that is to say, the education of the 
senses, The hand may become a most delicate instrument of 
observation; the eye may learn to see what it once could not; 
the ear to detect and discriminate what is quite beyond the 
uncultured hearing of the many. But it is one of the most 
convincing evidences of man’s superiority that in every 
field of observation he has risen above the lower animals, 
some of which by their unaided senses naturally excel him. 
So in this science, instruments have opened up mines of facts 
that must have otherwise remained hidden; they have, as 
it were, provided man with additional senses, so much have 
the natural powers of those he already possessed been sharp- 
ened. 
‘But the chief value of the results reached by instruments 
consists in the fact that the movements of the living body can 
be registered ; i. e., the great characteristic of modern physiol- 
ogy is the extensive employment of the graphic method, which 
has been most largely developed by the distinguished French 
experimenter Marey. Usually the movements of the point of 
lever are impressed on a smoked surface, either of glazed 
paper or glass, and rendered permanent by a coating of some 
material applied in solution and drying quickly, as shellac in 
alcohol. The surface on which the tracing is written may be 
stationary, though this is rarely the case, as the object is to get 
a succession of records for comparison; hence the most used 
form of writing surface is a cylinder which may be raised or 
lowered, and which is moved around regularly by some sort of 
clock-work. It follows that the lever-point, which is moved by 
the physiological effect, describes curves of varying complexity. 
That tracings of this or any other character should be of any 
value for the purposes of physiology, they must be susceptible 
of relative measurement both for time and space. This can be 
accomplished only when there is a known base-line or abscissa 
from which the lever begins its rise, and a time record which is 
usually in seconds or portions of a second. The first is easily 
obtained by simply allowing the lever to write a straight line 
before the physiological effect proper is recorded. Time inter- 
vals are usually indicated by the interruptions of an electric 
current, or by the vibrations of a tuning-fork, a pen or writer 
