322 Mr. H. F. Talbot's Experiments on Light. 



the stage of the microscope a plate of tourmaline, through 

 which the light of the large concave mirror is transmitted be- 

 fore it reaches the object lens. Another plate of tourmaline 

 is placed between the eyeglass and the eye ; and this plate is 

 capable of being turned round in its own plane, so that the 

 light always traverses both the tourmalines perpendicularly. 

 The mirror being adjusted so as to reflect the daylight into 

 the body of the microscope, and no object being as yet placed 

 on the stage, it follows, as indeed is obvious to all who are 

 conversant with this branch of optics, that if the two tourma- 

 lines are placed in a similar position, the light freely traverses 

 them both : but if that which is next the eye is turned round 

 90°, the observer can perceive nothing, except when the light 

 of the sun is used, which causes a small remnant of the light 

 to become visible. Except in this case, however, the field of 

 view is quite dark in this position of the tourmalines, and par- 

 tially bright in the opposite position. It is only partially, 

 and not entirely bright, because even the best tourmalines have 

 a considerable tinge of brown or green colour, which greatly 

 disturbs and disfigures the colours of all bodies which are 

 viewed through them. 



In consequence of this defect I found it advisable to abandon 

 the use of tourmalines, and to adopt, instead, an arrangement 

 of single-image calcareous spar, the invention of which is due 

 to Mr. Nicol. As I have examined the theory of this instrument 

 in another place*, I shall not revert to it at present further 

 than to say that the effect it produces is entirely similar to that 

 of the tourmaline, but that it possesses over that mineral the 

 precious advantage of perfect whiteness and transparency. 



This instrument has nearly the form of a four-sided prism 

 with a rhomboidal base, and it is conveniently placed in the 

 axis of a small brass tube, which is screwed on as close above 

 the eyeglass as possible, and forms a prolongation of the tube 

 of the microscope. A similar tube is screwed beneath the 

 stage of the microscope to transmit and at the same time po- 

 larize the light of the mirror. 



This arrangement is found to answer extremely well, for 

 when the two polarizing instruments are placed in a similar 

 position, the field of view is bright and perfectly white; but 

 when the one next the eye is turned round 90°, the field of 

 view is completely dark. It is in the latter situation that the 

 microscope is generally to be used. 



Having fitted up a microscope in the manner described, let 

 us take for the object of examination a hair. In order to make 



# See Lond. and Edinb. Phil. Mag., vol. iv. p. 289. 



