202 MEMORANDA. 
forms are, perhaps, the most beautiful in nature; but when 
we enter into their study, and penetrate somewhat deeply 
into the mysteries of crystallography, we meet with a very 
great drawback—the instability of certain crystals whose 
forms we find ourselves utterly unable to reproduce. Many 
of these endure for days, or even weeks, and then pass gradu- 
ally away; whilst with others a second crystallization always 
takes place, which robs the first form of all its beauty, or de- 
stroys it altogether. In these cases, when the form illus- 
trates some particular law, and it is desirable to preserve it, 
the pencil is usually employed. But some of these crystals 
exclude all chance of anything like a faithful drawing, by 
their intricacy of shape and the wearying labour that would 
be requisite to do them anything like justice. For these 
reasons I have been making use of photography by polarized 
light, and by this aid may be accomplished what could be 
done in no other way. In proof of this, I may mention that 
I have attempted by ordinary light a photographic impres- 
sion of certain salts; but I found it impossible, as nothing, 
except a faint trace of the outline, was produced upon the 
plate, which is easily accounted for by the excessive tenuity 
(and consequent transparency) of the crystals. Again, by 
the use of common light the peculiar characters of certain 
substances are totally lost, as the cross of starch, &c.; but 
by the aid of the camera and polariscope all these may be 
permanently produced upon paper. 
The power, also, of polarized light does not appear to be in 
any way inferior to that of ordinary light when used for pho- 
tographic purposes; but I hope to be able to make some 
more conclusive experiments in this matter shortly, as it is 
absolutely necessary to employ a light whose power is con- 
stant. Another curious fact which I have observed as to the 
power of polarized light as applied to photographic purposes 
is this: when the object upon the slide appears most per- 
fectly illuminated by the ray which has passed through the 
lower prism, we often find that the image in the camera is 
but partially distinct, and that we shall be forced to make a 
new adjustment of the mirror to procure an impression which 
will develop uniformly. 
As to the position, &c., of the microscope, there is little to 
say, as it used in the ordinary manner of photographing mag- 
nified objects which has been so often described. The re- 
sults, however, are better when the selenite plate is not 
employed, and the analysing prism is placed immediately 
over the object-glass. No. 1 eye-piece is, perhaps, the most 
preferable when the greatest distinctness is desired. 
