Illumination of Ohjects by Polariited Licjld. G3 
requisite pressure when any movement is to be registered, and 
to withdraw it while any other movements are in progress ; 
for it must be remembered that all movements made by the 
pencil, whether there be corresponding marks on the card or 
not, are traced on the glass so long as it is pressed against the 
diamond ; and, on the other hand, though the design be care- 
fully and accurately marked on the card, there will be no 
trace of it on the glass unless the lever has been properly 
employed. 
The object of this paper is to give a brief, but it is hoped, 
an intelligible account of the construction and manner of 
using a machine which has excited much interest, and not to 
consider the purposes to which its ingenious adaptations may 
be applied. At present the machine has been used almost 
solely for microscopic writing ; it is not, however, to be con- 
cluded that it can be used for nothing else. If, for example, 
the pencil, instead of being guided by the hand only, were 
directed and restrained by mechanical appliances, and the 
amount of the movements regulated by a screw or otherwise, 
there seems no reason why the productions of Nobert should 
not be rivalled. There would be little practical difficulty in 
causing the pencil to trace straight and parallel lines : regular 
distances of 1-lOOth of an inch are attainable. The power of 
the machine would diminish these intervals 6,000 times, so 
that a series of lines l-600,000th of an inch apart would be 
attainable. Whether a point could be got sufficiently fine to 
trace lines so close together, or any surface to receive them, 
may be questioned. If, however, such close ruling as this is 
impracticable, the limit would seem to be supplied by the 
want of a material to receive the tracings, and not to result 
from the insufficiency of the machine to execute the movements. 
Perhaps the machine might be made subservient to accurate 
dividing in the preparation of micrometers, &c., as by its 
means, errors or inequalities admit of being almost indefinitely 
diminished. 
On the Illumination of Objects hy Polarized Light on a 
Dark Field. By John Furze. 
(Read April 24th, 1855.) 
Allow me to call the attention of the Society to a method of 
viewing objects with the microscope, which is, I believe, novel 
in its arrangement, and no doubt admits of greater improve- 
ment than it has yet received at my hands, from the want of 
sufficient time and opportunity to devote to that purpose. 
The phenomena of polarized light have long added a charm 
