no less constant inconstancies by variation, were (and still continue,) mysteries reserved to later posterities." ART. XVIII.-Historical Account of Discoveries respecting the Double Refraction and Polarisation of Light. (Continued from Vol. III. p. 285.) PERIOD III.-Containing the Investigations of Newton, SECT. I.-Account of the Investigations of Sir Isaac THE only observations which Sir Isaac Newton appears to have published on the subject of double refraction and polarisation, are contained in the Queries printed at the end of the 3d Book of his Optics. As they are written with great perspicuity, and easily understood, we shall lay them before our readers in his own words. Query 25. Are there not other original properties of the rays of light, besides those already described? An instance of another original property we have in the refraction of Iceland Crystal, described first by Erasmus Bartholine, and afterwards more exactly by Hugenius, in his book De la Lumiere. This crystal is a pellucid fissile stone, clear as water or crystal of the rock, and without colour; enduring a red heat without losing its transparency, and in a very strong heat calcining without fu sion. Steeped a day or two in water, it loses its natural polish. Being rubbed on cloth, it attracts pieces of straws and other light things, like amber or glass; and with aqua fortis it makes an ebullition. It seems to be a sort of talc, and is found in form of an oblique parallelopiped,with six parallelogram sides and eight solid angles. The obtuse angles of the parallelograms are each of them 101°52′; the acute ones 78° 8'. Two of the solid angles opposite to one another,as C and E, (See'Plate III. Fig. 3.) are compassed each of them with three of these obtuse angles, and each of the other six with one obtuse and two acute ones. It cleaves easily in planes parallel to any of its sides, and not in any other planes. It cleaves with a glossy polite surface not perfectly plane, its but with some little unevenness. It is easily scratched, and by reason of its softness it takes a polish very difficultly. It polishes better upon polished looking-glass than upon metal, and perhaps better upon pitch, leather or parchment. Afterwards it must be rubbed with a little oil or white of an egg, to fill up scratches; whereby it will become very transparent and polite. But for several experiments, it is not necessary to polish it. If a piece of this crystalline stone be laid upon a book, every letter of the book seen through it will appear double, by means of a double refraction. And if any beam of light falls either perpendicularly, or in any oblique angle upon any surface of this crystal, it becomes divided into two beams, by means of the same double refraction. Which beams are of the same colour with the incident beam of light, and seem equal to one another in the quantity of their light, or very nearly equal. One of these refractions is performed by the usual rule of optics, the sine of incidence out of air into this crystal being to the sine of refraction, as five to three. The other refraction, which may be called the unusual refraction, is performed by the following rule, "Let ADBC represent the refracting surface of the crystal, C the biggest solid angle at that surface, GEHF the opposite surface, and CK a perpendicular on that surface. This perpendicular makes with the edge of the crystal CF, an angle of 19° 3'. Join KF, and in it take KL, so that the angle KCL be 6° 40', and the angle LCF 12° 23'. And if ST represent any beam of light incident at T in any angle upon the refracting surface ADBC, let TV be the refracted beam determined by the given proportion of the sines five to three, according to the usual rule of optics. Draw VX parallel and equal to KL. Draw it the same way from V in which L lieth from K; and joining TX, this line TX shall be the other refracted beam carried from T to X, by the unusual refraction. 1 "If, therefore, the incident beam ST be perpendicular to the refracting surface, the two beams TV and TX, into which it shall become divided, shall be parallel to the lines CK and CL; one of those beams going through the crystal perpendicularly, as it ought to do by the usual laws of optics, and the other TX by an unusual refraction diverging from the perpendicular, and making with it an angle VTX of about 68°, as is found by experience. And hence, the plane VTX, and such like planes which are parallel to the plane CFK, may be called the planes of perpendicular refraction. And the coast towards which the lines KL and VX are drawn, may be called the coast of an unusual refraction. “In like manner, crystal of the rock has a double refraction: but the differences of the two refractions is not so great and inanifest as in Iceland crystal. "When the beam ST incident on Iceland crystal, is divided into two beams TV and TX, and these two beams arrive at the farther surface of the glass; the beam TV, which was refracted at the first surface after the usual manner, shall be again refracted entirely after the usual manner at the second surface; and the beam TX, which was refracted after the unusual manner in the first surface, shall be again refracted entirely after the unusual manner in the second surface; so that both these beams shall emerge out of the second surface in lines parallel to the first incident beam ST. "And if two pieces of Iceland crystal be placed one after another, in such manner that all the surfaces of the latter be parallel to all the corresponding surfaces of the former: The rays which are refracted after the usual manner in the first surface of the first crystal shall be refracted after the usual manner in all the following surfaces; and the rays which are refracted after the unusual manner in the first surface, shall be refracted after the unusual manner in all the following surfaces. And the same thing happens, though the surfaces of the crystals be any ways inclined to one another, provided that their planes of perpendicular refraction be parallel to one another. And, therefore, there is an original difference in the rays of light, by means of which some rays are, in this experiment, constantly refracted after the usual manner, and others constantly after the unusual manner: For if the difference be not original, but arises from new modifications impressed on the rays at their first refraction, it would be altered by new modifications in the three following refractions; whereas it suffers no alteration, but is constant, and has the same effect upon the rays in all the refractions. The unusual refraction is therefore performed by an original property of the rays. And it re 3 mains to be inquired, whether the rays have not more original properties than are yet discovered. “Qu. 26. Have not the rays of light several sides, endued with original properties? For if the planes of perpendicular refraction of the second crystal, be at right angles with the planes of perpendicular refraction of the first crystal, the rays which are refracted after the usual manner in passing through the first crystal, will be all of them refracted after the unusual manner in passing through the second crystal; and the rays which are refracted after the unusual manner in passing through the first crystal, will be all of them refracted after the usual manner in passing through the second crystal. And, therefore, there are not two sorts of rays differing in their nature from one another, one of which is constantly and in all positions refracted after the usual manner, and the other constantly and in all positions, after the unusual manner. The difference between the two sorts of rays in the experiment mentioned in the 25th question, was only in the positions of the sides of the rays to the planes of perpendicular refraction. For one and the same ray is here refracted sometimes after the usual, and sometimes after the unusual manner, according to the position which its sides have to the crystals. If the sides of the ray are posited the same way to both crystals, it is refracted after the same manner in them both: But if that side of the ray which looks towards the coast of the unusual refraction of the first crystal be 90° from that side of the same ray which looks towards the coast of the unusual refraction of the second crystal, (which may be effected by varying the position of the second crystal to the first, and by consequence to the rays of light,) the ray shall be refracted after several manners in the several crystals. There is nothing more required to determine whether the rays of light which fall upon the second crystal, shall be refracted after the usual or after the unusual manner, but to turn about this crystal, so that the coast of this crystal's unusual refraction, may be on this or on that side of the ray. And therefore every ray may be considered as having four sides or quarters, two of which opposite to one another incline the ray to be refracted after the unusual manner, as often as either of them are turned towards the coast of unusual refraction; and the other two, whenever either of them are turned towards the coast of unusual refraction, do not incline it to be otherwise refracted than after the usual mánnrr. The two first may therefore be called the sides of unusual refraction. And since these dispositions were in the rays before their incidence on the second, third and fourth surfaces of the two crystals, and suffered no alteration (so far as appears) by the refraction of the rays in their passage through those surfaces, and the rays were refracted by the same laws in all the four surfaces; it appears that those dispositions were in the rays originally, and suffered no alteration by the first refraction, and that by means of those dispositions the rays were refracted at their incidence on the first surface of the first crystal, some of them after the usual, and some of them after the unusual manner, according as their sides of unusual refraction were then turned towards the coast of the unusual refraction from that crystal, or sideways from it. 66 Every ray of light has therefore two opposite sides, originally endued with a property on which the unusual refraction depends, and the other two opposite sides not endued with that property. And it remains to be enquired, whether there are not more properties of light by which the sides of the and are distinguished from one another. rays differ, "In explaining the difference of the sides of the rays above mentioned, I have supposed that the rays fall perpendicularly on the first crystal. But if they fall obliquely on it, the success is the same. Those rays which are refracted after the usual manner in the first crystal, will be refracted after the unusual manner in the second crystal, supposing the planes of perpendicular refraction to be at right angles with one another, as above, and on the contrary. "If the planes of the perpendicular refraction of the two crystals be neither parallel nor perpendicular to one another, but contain an acute angle, the two beams of light which emerge out of the first crystal, will be each of them divided into two more at their incidence on the second crystal. For in this case the rays in each of the two beams will some of them have their sides of unusual refraction, and some of them their other sides turned toward the coast of the unusual refraction of the second crystal." |