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III. On the Optical Phenomena of certain Crystals. By H. F. Talbot, Esq. F.R.S * 
Received April 20, — Read May 5, 1836. 
SOME time ago I had the honour to communicate to the Royal Society an account 
of my invention of the polarizing microscope -f\ This instrument possesses so great 
a power of developing the internal structure of transparent bodies, even in their mi- 
nutest visible particles, that I feel confident the employment of it will lead to many 
new and interesting results. At present I mean to confine myself to the description 
of a phenomenon which shows strikingly the beautiful order and regularity with 
which nature disposes the fabric of some of her minutest visible works. 
The object I speak of is a kind of minute crystallization which may be obtained 
in peculiar circumstances, and I doubt not, in many different ways ; but the manner 
in which it has presented itself to my observation is as follows. 
A crystal of borax is placed in a drop of phosphoric acid, somewhat diluted, upon a 
plate of glass,and then moderately heated until thecrystaldissolvesinthe acid. Itis then 
set aside to crystallize. It is well to prepare a number of these plates at once, varying 
the relative proportion of the acid and salt, in order that the desired kind of crystal- 
lization may be found in one or other of them ; for there is a considerable variety in 
the crystalline forms obtained by this method, some of which indeed are very sin- 
gular. But when that kind of crystallization takes place which it is more particu- 
larly my intention to speak of, the field of view of the microscope is seen covered 
with minute circular spots, each of which is like a tuft of silk radiating from a centre, 
and is composed of a close assemblage of delicate acicular crystals forming a star. 
But besides these, are seen interspersed among them a number of circular transparent 
bodies, which are evidently modifications of the former, being, in fact, tufts or stars 
of acicular crystals in such close assemblage as to be in optical contact with each 
other and to produce the appearance of a single individual. Now let us suppose a 
group of these circles to be under examination with the polarizing microscope, and 
when the polarizers^ are crossed, we observe the following phenomenon. The field 
* This paper appeared in the Philosophical Magazine for October 1836, after it had been ordered by the 
Council to be printed in the second part of the Philosophical Transactions for that year. The circumstance 
arose entirely from a mistake of the person to whom the superintendence of the printing both of the Transac- 
tions and Magazine is entrusted, and neither Mr. Talbot, the Council, nor the Officers of the Royal Society had 
any cognizance of the error till the paper appeared in the latter publication. 
t See London and Edinburgh Philosophical Magazine, vol. v. p. 321. 
+ By this term, for the sake of brevity, I here designate the polarizing and analysing prisms of single-image 
calcareous spar, or the plates of tourmaline which may be employed in their stead. 
MDCCCXXXVII. E 
