Page 403. 
66 ON SPHYGMOGRAPHY. 
little practice removes all difficulty. It is worthy of notice that better 
traces can be obtained after the instrument has been applied a few 
minutes than immediately; for the pad, by its pressure on the skin, 
drives blood from the small vessels covering the artery, and so lessens 
the distance between it and the pulse-spring. 
To record the movements of the lever, it is best to employ highly 
glazed paper which has been smoked thinly by the flame of a compo- 
site candle; and to effect this properly, the paper must be folded on 
the metal plate, which is connected with the running gear of the 
sphygmograph, and moved quickly up and down in the flame. By 
this means its edges are not charred, and a more uniform film is pro- 
duced. The pen must have a fine sharp point. A spirit varnish, such 
as is used for photographic plates, fixes the tracings, if a little is 
poured over them gently, and the paper is subsequently warmed. 
An entirely novel form of sphygmograph has been constructed by 
M. Longuet,* which, from a figure in Dr. Loraine’s work, appears to 
possess all the requirements of an accurate instrument, except in the 
recording portion, where the pen, instead of writing laterally, as it 
might easily be made to do, rests on the paper horizontally, and has 
to change its relation to the body of the instrument whenever the pulse- 
pad moves in the least. In principle its construction is in a great 
measure similar to that of the cardiograph in my combined cardio- 
sphygmograph, which was described in this Journal in May, 1871.+ It 
has a dynamometer attached, which measures the pressure and its 
changes throughout the beat. 
One of the points in which Marey improved upon Vierordt’s original 
sphygmograph was, that he made his lever write laterally instead of 
at its tip, by which means he obviated all difficulties connected with an 
exact up and down movement of the recording pen. But this gives 
rise to another slight error, which can and must be corrected in com- 
paring the length of the elements of the pulse-beat. The lever, when 
the clockwork is stationary, traces a curved line and not a straight 
one, the curve being part.of a circle, of which the lever is the radius, 
and its arbor the centre. Therefore, when the watchwork is in action, 
the horizontal relation of points at different heights on the recording 
paper is not truly represented by straight lines, drawn perpendicular 
to the length of the paper, but by lines projected from these points in 
curves parallel to that produced by the lever on the paper when it is 
stationary. These lines can be easily formed after the tracing has been 
removed from the apparatus, by scratching on it with a needle that is 
tied to a nail, or a piece of board with two pieces of cotton of the 
* “ Bulletin de l’Acad. de Médecine,” XXXTIT. p. 962, 1868. 
+ Supra, p. 27. 
