transmitted through crystallized Bodies. 269 



fee considered as a new case of the production of colour ; and 

 though we do not pretend to point out its cause, yet it obviouslv- 

 depends upon a particular structure which is posssessed only by 

 6ome portions of the agate, and admits of such variations as to 

 produce the same colours at diiferent angles of incidence. 



IV. On the Depolarization of Light. 



In the fourth book of my Treatise on new Pliilosophical Instru- 

 ments, I have already shown that almost all transparent crystals 

 possess in two positions the singular faculty of depolarizing light, 

 or of depriving it of the property which it acquires bv trans- 

 mission through the agate, while in other two positions of the 

 depolarizing crystal, the polarity of the light suffers no change. 

 Thus in Plate IV., fig. 5, let ABDC be a piece of mica or of any 

 other crystallized body interposed between a plate of agate and 

 a prism of Iceland spar when one of the images has vanished, 

 and let GH be parallel or perpendicular to the laminae of the 

 agate when the vanished image continues invisible. This line 

 I have called the neutral axis, as no effect is here produced upon 

 the polarized light. By turning the mica round, the vanished 

 image will gradually appear ; and when the line AD comes into 

 a vertical position, it will be restored to its full lustre, and will 

 never again vanish, whatever be the position of the Iceland spar. 

 The line AD I have therefore called the depolarizing axis, as 

 the light in passing through it has been deprived of the polarity 

 commmiicated by the agate, and which prevented it from pene- 

 trating the rhomboid of Iceland spar. 



By continuing the motion of the mica, it will be found tliat 

 EF is also a neutral axis, and BC a depolarizing axis. The 

 depolarizing axes are common to almost all crystallized sub- 

 stances; and u'liat is very singular, I have discovered them in 

 horn, gum arabic, glue, tortoihe-.sliell, caoutchouc, goldbeater's 

 skin, amber, mother of pearl, camiihor, spermaceti melted and 

 cooled, bees' wax melted and cooled, adipucire melted and 

 cooled, manna, oil of mace, acetate of lead melted and cooled, 

 human hair, bristles of a sow, human cornea, cornea of a fish, 

 cornea of a cow, and imperfectlv in some pieces of plate glass. 



Plates of mica, however, while they possess the properties of 

 all depolarizing crystal-^, exhibit ph.vnomena peculiar to them- 

 selves. If the neutral axis GH of a plate of mica is inclined 

 forwards so as to make a considerable angle with the horizon, 

 the image that was formerly invisii>!e will start into existence, 

 and therefore the ncntral axis GH is accompanied with au 

 oblique de|)olarizing axis Nn. Tliis oblique axis is also pos- 

 sessed by topaz, rock crystal, and many otlier crystaliized bodies. 

 In making the same experiment with the depolarizing axis of 



the 



