July II, 1889] 



NATURE 



257 



I 



OPTICAL TORQUE} 

 II. 



T will be convenient here for me to refer to some researches, not 

 yet published, which I have made, as to the various orders of 

 transition tints, with the view of ascertaining which of them is 

 the most sensitive— which of them, in fact, shows the greatest 

 change of tint for the smallest amount of rotation. Reference 

 to the diagram on the wall displaying Newton's tints will make 

 clear what I mean by the transition tints of the several orders. 

 The tints obtained from quartzes of varying thicknesses may be 

 considered as approximately identical with the tints of Newton's 

 rings, provided we remember that the air-film which gives any 

 particular tint in Newton's rings is about 1/300,000 part as thick 

 as the quartz which yields the corresponding tint in the polari- 

 scope. Better far than any painted diagram, because richer and 

 purer, are the tints now thrown upon the screen by introducing 

 into the field a thin wedge of selenite, displaying the whole of 

 the colours of the first three orders of Newton's scale. You will 

 notice the successive recurrence of purple tints, both in the 

 colours seen in the bright field, and in those seen in the dark 

 field. 



First I will show you the transition tints of the first and 

 second orders in the bright field. That of the second order is 

 much less intense than that of the first ; and yet it is very sensi- 

 tive, turning to a green tint whilst the first order purple has only 

 turned to a blue. On the other hand, with reversed rotation of 

 the analyzer it turns to red less rapidly than does the tint of the 

 first order. 



Next I take the transition tints of orders I., II., and III. in 

 the dark field. These, though arranged, by means of superposed 

 half-disks of " quarter- wave " plates, to be optically equivalent 

 to biquartzes of two rotation?, are really built up of selenite and 

 mica. You will notice how the tint of order I. surpasses in 



sensitiveness both the others. I cannot here show you on the 

 screen the means by which I have compared the tint of order I. 

 in the dark field with that of order I. in the other set. Suffice it 

 to say that I find the tint of order I. in the dark field— corre- 

 sponding to 7-5 millimetres thickness— more sensitive than that 

 of order I. in the bright field, which corresponds to 375 

 millimetres thickness. 



A method which was at one time supposed to be more precise, 

 was that of placing a spectroscope (or its prism) in front of the 



Fig. 9. — Direct-vision prism for projection of spectrum. 



analyzer, and watching the motion along the spectrum of the 

 interference bands which are then seen. My three pieces of 

 crystal remain. I introduce a slit in front of them, also a single 

 film of quarter-wave mica, and then a prism to give the spectrum. 

 This prism (Fig. 9), by the way, is a new sort of direct-vision 

 prism, having a single very wide-angled prism of Jena glass 

 inclosed in a cell with parallel ends containing cinnamic ether 

 (first recommended by Wernicke), a liquid which has the same 



R 



V 



Fig. 10. — Direct-vision prism, a, wide-angled prism of Jena glass ; c, cinnamic ether. 



mean refractive power, but widely different dispersion. It is 

 preferable to bi-ulphide of carbon in several respects : first, its 

 odour is a delicate reminiscence of cinnamon ; it is barely vola- 

 tile ; and it is whiter than bisulphide. This prism, which is 

 shown also in plan in Fig. 10, was constructed for me by Messrs. 

 R. and J. Beck. It will be seen that the dark bands in the 

 spectrum are nebulous and ill-defined. It is idle to hope to 

 secure accuracy by turning the analyzer until they shift along to a 

 definite point. And there is no advantage in using the higher 

 orders of tints which give more bands ; for, though the bands are 

 certainly better defined, their progression across the spectrum 

 for a given amount of rotation is proportionally smaller. 



Another suggestion, due to Senarmont, is to use two sets of 

 superposed wedges of right- and left-handed quartz. Such you 

 now see before you. Instead of starting with extinction you 

 start with coincidence between the upper and lower set of bands. 

 Any rotation of the light shifts the bands, one set moving to 

 left, the other to right. By turning the analyzer through an 

 equal angle coincidence is again obtained. 



Another method, used by Wild in his polaristrobometer, is to 

 produce the phenomenon known as Savart's bands (due to the 

 introduction of two crossed slices of quartz cut at a particular 

 angle). The bandsdisappear when the analyzer is set in a particular 

 direction. Anything that twists the plane of polarization causes 

 them to reappear ; but they again fade out when the analyzer is 

 turned through an equal angle. 



There is yet another method in polarimetry, due to Soleil, in 

 which the optical torsion due to the sugar is counterbalanced or 

 compensated by introducing a pair of sliding wedges of quartz 



' A Discourse delivered at the Royal Institution, May 17, 1889, by Prof 

 Silvanus P. Thompson. Continued from p. 235. 



of the opposite rotation. This device is known as a "com- 

 pensator." By sliding the quartzes over one another a greater 

 or less thickness of quartz is introduced at will. But I must not 

 stop to illustrate this elegant device. 



Yet one other method must be mentioned, and this is certainly 

 the most preferable. It consists in aiding the eye to recognize 

 with precision a particular degree of extinction, by the device, 

 first suggested in 1856 by Pohl, of covering a portion of the 

 visible field with something which slightly alters the initial plane 

 of polarization, so that complete blackness is not obtained at 

 once over both parts of the field. A common device is to cover 

 half the field with a slice of some thin crystal — mica or quartz — 

 so that only one talf can be perfectly black at any instant. As 

 an example, here is the field covered half over with a plate of 

 mica of the thickness known as half-wave. The result is that 

 when one half of the field is black the other is light. Adjust- 

 the analyzer now to equality. Now introduce something that 

 rotates the light— say a tube with sugar solution in it. At once 

 the balance is upset, and I must, in order to get equality, turn 

 my analyzer. 



Of the same class are the polarimeters with special prisms 

 made in two parts slightly inclined to one another. The earliest 

 of these was devised by the late Prof. Jellett, of Dublin, and 

 has been followed by imitations of the same plan by Cornu, by 

 Lippich, and by Schmidt and Haensch. The beautiful " shadow 

 polarimeter," by the latter firm, which I here exhibit, has the 

 divided prism, and a quartz compensator. 



I have suggested two simpler methods of accomplishing the 

 same end. In the first place, I have proposed to use iwin-prisms. 

 These are made on a plan suggested to me by finding that Mr. 

 Ahrens's method of cutting calc-spar for prisms was admirably 



