368 Miscellaneous Intelligence. 
that at this moment asimple, cheap, and accurate dynameter, is ra- 
ther a desideratumm ; the best, 1 believe, now in use is that invented by 
the late ingenious Ramsden, whose ruling passion seems to have 
been not only to surmount difficulties, but to create them also in 
many instances. He seems to have selected one of the most com- 
plicated and difficult principles to carry into effect on which a dyna- 
meter can be formed; and however excellent it may be in itself, very 
few workmen of the present day will undertake to execute dyname- 
ters of his construction. In consequence the most common instru- 
ment of the kind is nothing more than a mother-of-pearl micro- 
meter, with divisions of an inch into 200 parts, attached to a lens. 
This again is too coarse an instrument, and is, moreover, very difficult 
to use, having no contrivance to adjust it to perfect vision on the 
pencil of light, in addition to which it frequently cannot be adapted 
to measure high powers at all, from an impossibility of getting it 
close enough to the eye-picce, the brass work of which will not per- 
mit the plate of the micrometer to arrive at the point on which a 
very short pencil of rays falls. ‘To obviate all these inconveniences, 
nothing more would be necessary than to use a compound micro- 
scope, having the micrometer at its field-bar, in the focus of the 
eye-glass. It will be very easy to shew that this sort of dynameter 
will be perfectly commodious, not liable to get out of order, and 
susceptible of any degree of accuracy which we may think it neces- 
sary to obtain; I am only surprised that it is not to be found in all 
the opticians’ shops. 
Let us suppose the object-glass of such a microscope to be 4 inch 
focus, that the eye-glass is 1 inch focus, with a negative field-glass, 
and that there is a micrometer of mother-of-pearl at the field-bar 
having divisions to the =45 of an inch, (which I know by experi- 
ence can be read with a lens of 1 inch focus).—According to the 
length of the tube of the microscope the image at the field-bar will 
be more or less magnified,—say it is 7 times larger than the object— 
then a pencil of rays of =$5 of an inch diameter will subtend 75 
on the micrometer, and may be seen divided into 7 parts, therefore 
it may be measured to the 3555 of an inch, a degree of accuracy 
quite sufficient, I apprehend, for practical purposes—if not, we have 
only to increase the depth of the object-glass, and we may obtain a 
scale to any extent we please. In the same manner, if the divisions 
of the micrometer are not seen with sufficient ease, the depth of the 
eye-glass may be augmented. Were it an object to carry this prin- 
ciple to its utmost extent, onc of Mr, Troughton’s micrometers might 
be attached to the body of the microscope; but this | apprehend 
would be quite superfluous. 
One circumstance in constructing this dynameter must be strictly 
attended to; I mean the ascertainment of the exact value of the di- 
visions of the mother-of-pearl, which is done with perfect facility 
by viewing another similar micrometer placed in the focus of the 
