180 Mr, Ilerschel on the Action of [March^ 



I have thus occasionally examined the rings in a portion not 

 €xceedin^ the hundredth of an inch in diameter, and thus 

 detected irregularities of crystallization of a very singular nature, 

 in many bodies, which would have eluded any other mode of 

 .observation. For this purpose the crystal must be cemented 

 over a small aperture in a thin sheet of brass, on which the focus 

 of the lens must be exactly adjusted to fall.* 



If, instead of applying the eye to receive the light so dis* 

 persed, we place a screen at some distance in a darkened room,, 

 the apparatus is converted into a solar microscope, and the rings 

 will be seen projected on the screen. The construction of the 

 apparatus I employed is as follows : A B is a brass tube, within 

 which are fitted, first, a fixed diaphragm, a a b b, carrying the 

 first plate of tourmaline in its centre ; secondly, a diaphragm, 

 cc ddy moveable freely in its own plane by means of the pin g 

 passing through a slit in the i ide of the cylinder, A B, which 

 .occupies an arc of about 120° of its circumference. This is des- 

 tined to receive the crystallized plate, d d, while a cylinder,. 

 ,hh e eff, made to slide and turn smoothly within A B carries 

 .the second tourmaline, y]/'. It is essential that the tourmalines 

 employed for this purpose, and especially the posterior one, 

 should be perfectly free from all flaws and blemishes ; but large 

 plates not being required, this condition is easily satisfied. The 

 plates so arranged, and brought as near together as possible, the 

 extremity A of the cylinder A B is fitted to slide somewhat 

 stiffly on the brass tube P Q, furnished with a lens L, of about 

 .two inches focus, and a screw P P, by which it can be adapted 

 ,to the apparatus usually employed for reflecting a sunbeam into 

 ; a darkened chamber. The sliding motion of the cylinder A B 

 • allows the focus of the lens to be adjusted so as to fall exactly 

 on the first surface of the posterior tourmaline /*, while its rota- 

 .tion suflPers the axis of the anterior one to be placed perpendi- 

 cular to the plane of reflection. By this arrangement two 

 advantages are gained. The reflector employed (though metalhc) 

 always polarises a more or less considerable portion of the 

 . reflected beam, which in any other position is partially or totally 

 . extinguished by the first tourmaline, and a great loss of light 

 ensues, which it is of the utmost consequence to avoid : more- 

 over, by this disposition, the action of the reflector is brought to 

 . conspire with that of the tourmaline, and the polarisation of the 

 light which traverses it (which is never rigorously exact) is 

 thereby reixdered more complete. 



It is convenient to have sHding tubes containing lenses of dif- 

 ferent focal lengths, according to the crystal examined, for the 



; • I have now an apparatus preparing, in which the first plate of tourmaline itself is 

 formed into a double convex lens, by which the loss of light at two surfaces will be sup. 

 pressed. It is easy to adapt such a lens to a double microscope, for the purpose of detect- 

 ing microscopic irr^ularities ; and I have reason to suppose a variety of curious results 

 will be brought to light by these meanji. 



