Mechanical and General Science. 369 
J 
object-glass ; and by shortening or lengthening the tube of the mi- 
croscope, the divisions may be made to coincide in any point which 
is selected. J think it will be found convenient to have the micro- 
meter in the field-bar on a narrow slip of mother-of-pearl divided into 
100 parts to one inch, and then again into 5 more, and to adjust the eye- 
glasses and the length of the tube so that 1, of an inch in the focus 
of the object-glass shall be equal to 1 inch at the field-bar, and so to 
fill the whole of the field of view. It would perhaps facilitate the 
reading of the divisions if a dot were placed at every tenth of an inch 
on the micrometer. 
With respect to mechanical arrangements, the body of the dy- 
nameter should be made to slide up and down in another tube with 
or without rack work, which may be pressed firm by the hand 
against the eye-piece of the telescope, whose powers it is applied to 
measure ; while the internal microscope is adjusted to distinct vision, 
the external tube may be casily made applicable to any telescope, or 
a shoulder with a screw might be left upon every eye-piece to which 
the said tube may be firmly attached. It will be evident that the 
object-glass of such a dynameter will always be at an abundant 
distance from the shortest pencil of rays it is employed to measure. 
I should scarcely have thought it worth while to have pointed out 
so obvious an application of the compound microscope, but I have 
never seen or heard of its adaptation to any such purpose as I have 
recommended, Jt must be recollected that some have the faculty of 
perceiving things for themselves, others only when they are pointed 
outto them, and many hardly then ;—of this the history of Columbus 
and hisegg will remind us. 
II. Cuemicat Scrence. 
1. On a Reciprocity of insulating and conducting Action which the 
incandescent Platina of Davy exerts on the two Electricitics.—The fol- 
lowing is part of an extract communicated to the Annales de Chimie, 
(xxy. 278.) from a memoir of M. Erman, inserted in the memoirs of 
the Academy of Berlin, for the years 1818 and 1819. 
Place on an electrometer an aphlogistic lamp, of which the upper 
spirals of platina wire are in full incandescence, and hold at the 
distance of four or five inches above the lamp the negative pole of a 
dry voltaic pile, or the negative coating of a small Leyden jar 
feebly charged, the electrometer will diverge powerfully. Present in 
the same manner the positive pole or coating, there will be no diver- 
gence or atleast a very slight one, and that due to induction, 
Place above an insulated aphlogistic lamp, at the distance of four 
or six inches, a small screen of any conducting substance, making it 
communicate with an electrometer, then touch the lamp with a po- 
sitive pole or coating, ad the electrometer of the screen will diverge 
