36 Mitchell^ on a Pulsatile Muscular Organ. 
methods in use require much previous calculation. That 
none is necessary in Mr. Lister's^ either in drawing the scale 
or in using it, is evident ; and the value of the divisions in 
every form of eyepiece micrometer_, whether cobweb or glass, 
is obtained by observation, and can be so adjusted by means 
of a draw- tube, that the only calculation required — the multi- 
plying or dividing by a single figure — may in all cases be 
performed mentally. 
The author's method, however, is founded on correct prin- 
ciple, will give a tolerable approximation when carefully used, 
and can readily be adopted by observers in any " out-of-the- 
way corner of the world." On these grounds the Council of 
the Microscopical Society is recommended to give it a place in 
their Transactions.' 
The referee does not, however, concur in the author's esti- 
mate of its extreme accuracy ; which he appears to found on 
the minuteness to which the divisions of the scale can be 
carried, without considering the difficulty of observing their 
ea^act coincidence with the image of the object to be measured 
when the slightest movement of the head must alter their re- 
lative position. 
Nor would he wish the Council to recommend instrument- 
makers to print scales for different magnifying powers; because 
the length of the body will materially affect the result, unless 
they are placed at the definite distance of ten inches from the 
eye. 
For an observer, situated like Mr. Gibbons, at an immense 
distance from any microscope-maker, this form of micrometer 
will form a respectable substitute, until he can procure some- 
thing more accurate. — George Jackson, Referee.] 
On a Pulsatile Muscular Organ, auxiliary to the Circu- 
lation, found in the Legs of certain Insects. By Lieut. 
J. Mitchell. 
(Read Dec. 22d, 1858.) 
This organ, which was found in all the legs of an insect 
belonging to the family Nepadse, is a pulsating muscular 
sac, the nature of whose office is made apparent by the fact 
that the circulation of the blood ceases the instant its motion 
is interrupted. The pulsation being intermittent, these 
interruptions are frequent; the action continues for about 
ninety seconds and then ceases for about half that time. The 
movement of the blood-discs is always preceded by the pulsa- 
