191;;] on Applications of Polarized Light 733 



extinction. When plates of suitable thickness were used the trans- 

 mitted light was no longer white, but beautifully tinted, the colour 

 of the light varying with the thickness of the plate. Thus with 

 increasing thickness we have progressively yellow, orange, rose-red,, 

 violet, blue, and green. 



These colours were shown by Biot to be due to a rotation of the 

 plane of polarization, which increased (a) with the thickness of the 

 plate, (b) with change of colour from red to violet. It is therefore 

 impossible when a Ijeam of polarized light has passed througli a 

 quartz plate to extingnish all the colours simultaneously. 



The tints which Arago observ'ed were due to the selective ex- 

 tinction of light of different colours by the mirror which he used as 

 an analyser. This selective extinction may be shown by inserting a 

 direct vision spectroscope in front of the apparatus : the plate which 

 produces the pale yellow colour has rotated the violet light through 

 180°, so that it is extinguished exactly as if no quartz plate were 

 present ; the yellow tint is the complementary colour to that ex- 

 tinguished. As the thickness of the plate increases, the same effect 

 is produced with light of longer wave-length ; as the extinction 

 moves from violet to red the complementary colour changes from 

 yellow to orange, red, blue, and green. "When the bright yellowish- 

 green is extinguished a grey " neutral tint " is produced which is 

 extremely sensitive to small rotations of the plane of polarization, 

 and was at one time used very largely in polarimeters illuminated 

 with white light. 



When the mercury arc is used as a source of light, the colours are 

 mainly two — green and violet ; but the violet colour, especially, is so 

 beautiful that I cannot refrain from showing you a few of Spottis- 

 woode's experiments as they appear when the light of a mercury arc is 

 substituted for the white light which he used in illustrating his 

 Friday evening discourses. 



When monochromatic Hght is used — as, for instance, when a green 

 screen is placed in front of the mercury arc — the light can be extin- 

 guished completely even after it has passed through a very long 

 column of quartz. Using green light purified by a spectroscope and 

 rods of quartz cut from a crystal of extraordinary beauty, I have 

 obtained a perfectly sharp extinction with a column of quartz half a 

 metre in length, giving an actual rotation of 12,78'.»'20° ± 0'01°. 

 I have also been making experiments with the same material to 

 determine accurately what rotation is produced by quartz in light 

 of different wave-lengths, not only in the visible spectrum, but also 

 in the infra-red and ultra-violet regions ; but as the work is still 

 incomplete, I will not attempt to descri))e it. but pass on at once to 

 other ways in which rotatory polarization may be produced. 



Three years after his discovery of rotatory polarization in quartz, 

 Biot was astonished to find that the same property was possessed 

 by certain liquids, turpentine and laurel-oil rotating the plane of 



