288 Mr. Talbot on the Optical 



Mr. Rainey can increase the power of a horse-shoe magnet of 

 tempered steel, which has been magnetized in the usual way, to 

 the same strength as the 'weakened magnet, by means of per- 

 fectly soft iron, then it must beachnitted that the Hfter can by 

 \lsreactio)i induce a greater degree of magnetism than the prime 

 motor itself })ossessed. Till then we must admit the truth of 

 the Newtonian law of action and reaction without exception 

 or reservation. 



s 



LX. On the Optical Phccnomena of certain Crystals. 

 Bii H. F. Talbot, Esq., F.R.S*- 



OME time ago I had the honour to communicate to the 

 Royal Society an account of my invention of the polarizing 

 microscope t- This instrument possesses so great a power of 

 developing the internal structure of transparent bodies, even 

 in their minutest visible particles, that I feel confident the em- 

 ployment of it will lead to many new and interesting results. 

 At present I mean to confine myself to the description of a 

 phsenomenon which shows strikingly the beautiful order and 

 regularity with which nature disposes the fabric of some of 

 l)er minutest visible works. 



The object 1 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 some- 

 what diluted upon a plate of glass,and then modeiately heated 

 until the crystal dissolves in the acid. It is then set aside to cry- 

 stallize. 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 crystallization may be found in one or other 

 of them ; for there is a considerable variety in the crystal- 

 line forms obtained by this method, some of which indeed are 

 very singular. But when that kind of crystallization takes 

 place which it is more particularly my intention to speak of, 

 the field of view of the microscope is seen covered with mi- 

 nute circular spots, each of which is like a tuft of silk radi- 

 ating 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 trans- 

 parent bodies, which are evidently modifications of the former, 



* Read before the Rojal Society May 5th, 1836 : and now communicaled 

 by the Author. 



1 See Lond. and Ediiib. Philosophical Magazine, vol. v. p. 321. 



