2 34 



NATURE 



[July 4, 1889 



■of hilf a degree. I should say that the whole of this model 

 has been constructed by my assistant, Mr. Eustace Thomas. 



Now let me show you the optical effect which corresponds to 

 this. Placing a second Nicol prism as analyzer in the path 

 of the polarized waves, I turn it to the position where it cuts 

 off the polarized light. The " dark field" so produced by the 

 crossed Nicol prisms corresponds to the motionless cord beyond 

 the crossed analyzer of the acoustic apparatus. 



Returning for a moment to two well-known forms of polarizing 

 apparatus, viz. the black glass reflector and the Nicol prism, I 

 may be permitted to refer to some recent attempts to improve 

 upon these devices. 



Fig. 4. — Nicol prism : original form. 



The Nicol prism, as is well known, consists of a rhomb of 

 Iceland spar cut into two pieces, which are reunited by a film of 

 Canada balsam. As originally devised, it had oblique end faces 

 (Fig. 4), and a comparatively narrow angle (19°) of aperture. 

 These may be noticed in the small example which I here exhibit, 

 which is an original constructed by William Nicol himself. It 

 also has the disadvantage of giving a field in which the directions 

 •of the planes of polarization are not strictly parallel to one 

 another throughout its whole extent. Consequently there is 

 never complete extinction of light all over the field at one time. 

 Hartnack and others have attempted to remedy this by giving the 

 prism a different form and using other materials than Canada 



Fig. 5. — S. P. Thompson's modification of the Nicol prism. 



balsam. I have from time to time made many attempts to im- 

 prove upon the original construction. First, I have made the 

 end faces principal planes of section (Fig. 5) ; secondly, I have 

 made the axis of vision cross the crystallographic axis at right 

 angles, so getting a flatter field, a shorter length, a wider angle, 

 and less loss of light by reflection. Mr. Ahrens, the prism-cutter, 

 on whose able assistance I have relied during the last six or seven 

 years in cutting these prisms, has aided me with his ingenuity in 

 devising a method of cutting up the spar so as to give these ad- 

 vantages with a minimum waste of material. He has further 

 devised a method of putting a polarizing prism together in three 

 instead of two pieces — illustrated in the diagram (Fig. 6) — which 



Fig. 6. — Ahrens's triple prism. 



gvves a still wider angle. The prism which I shall use as analyzer 

 in the next experiments is one of these forms. 



Unfortunately at present there is a spar-famine, pieces of 

 Iceland spar of a size and purity suitable for the making of large 

 polarizers such as that I employ being not now procurable at 

 :any price. To avoid the excessive cost of large Nicols I have 

 lately got Mr. Ahrens to construct for me a large reflection- 

 polarizer, on the plane of Delezenne, but modified by Mr. 

 Ahrens in detail. In this prism the light is first turned 

 to the proper polarizing angle by a large total-reflection prism 

 of glass, and then reflected back, parallel to its original path, 



by impinging upon a mirror of black glass covered by a single 

 sheet of the thinnest patent plate-glass to increase the intensity 

 of the light. This form of polarizer, depicted in Fig. 7, is quite 

 equal for projection purposes to a Nicol prism of equal aperture, 

 and is much less costly. This one has 2^ inches clear aperture. 



Having so far reviewed the apparatus for polarizing and 

 analyzing, I will return to the apparatus set with its prisms 

 crossed, so that the analyzer completely extinguishes the polarized 

 light emitted from the polarizer. 



If in the space between polarizer and analyzer anything 

 be introduced which can either resolve obliquely the polarized 

 vibrations or twist them bodily round, then there will not be 

 complete extinction ; the amount of light passing the analyzer 

 depending in the one case on the obliquity of the resolution, in 

 the other upon the degree to which the vibrations are twisted or 

 rotated upon themselves. 



The effect of oblique resolution I may illustrate by introducing 

 a slice of tourmaline between the crossed Nicols, and rotating it 

 till it stands at 45° ; or, in the acoustic model, by introducing an 

 oblique pair of guide-pins. 



The other case — namely, that of producing a bodily twist of 

 the vibrations, rotating the plane of polarization around the 



Fig. 7. — Ahrens's reflecting polarizer. 



path of the wave — is not so easily illustrated by the model. 

 But it is optically perfectly simple : all that is requisite is to 

 introduce between the crossed Nicols a thin slice of that crystal 

 — namely, quartz — in which this effect of rotating the jjlane of 

 polarization was first observed. 



I take a clear plate of quartz, just i millimetre in thickness, 

 and interpose it between the crossed Nicol prisms. You will 

 note how the introduction of this plate of quartz brings some 

 light into view. 



Suppose we now turn the analyzer to try and obtain extinc- 

 tion : we get tinting. If we put in a coloured glass so as to 

 work with one kind of light only, we shall get extinction at a 

 particular angle. The table of data to which I invite your 



Optical Torsion produced by Plate of Quartz. 



I millimetre. 375 millimetres. 



