REFRACTION. 



833 



tinct creed, composed by Episcopius. In France, 

 the Reformed party (see Huguenots) was exposed 

 to the severest attacks ; by the edict of Nantes, in 

 1598, they first enjoyed toleration. But although 

 they submitted to the doctrines of Calvin, they had 

 no firm bond of union within themselves. The Eng- 

 lish church, which is reckoned among the Reformed, 

 was instituted in so peculiar a way, was involved 

 so early in controversies among its members, and 

 divided into so many parties, that it hardly deserves 

 thg name of a church. The creed of 1551, which 

 consisted originally of forty-two articles, but was 

 reduced to thirty-nine articles, in 1562, by the synod 

 of London, and was neither purely Zuinglian nor 

 Calvinistic, could not unite the contending parties. 

 (See England, Church of.) Unsuccessful attempts 

 have often been made to unite the Reformed and 

 Lutheran churches. In the Prussian dominions, and 

 some other German states, however, since 1817, the 

 long projected amalgamation of the Reformed and 

 Lutheran churches into one evangelical Christian 

 church has been partly effected, and attended with 

 important results. 



REFRACTION, DOUBLE; a department of the 

 science of optics, which owes its origin to the ob- 

 servations of the illustrious Huygens. In looking 

 through a piece of Iceland or calcareous spar (car- 

 bonate of lime), he was surprised to find that a 

 double image of the object was refracted by the 

 crystal in certain positions, the rays of one image 

 following the ordinary law of refraction, dis- 

 covered by Snellius, and explained under Refrac- 

 tion in our article Optics, but the rays of the other 

 image following another law. 



Iceland spar, or carbonate of lime, is found 

 generally in crystaline rhomboids, the angles of the 

 sides being 105. It is transparent, susceptible of a 

 fine polish, and has a specific gravity of 2 - 714. The 

 cleavage of the crystal is easily effected, and the 

 face after cleavage is generally planed and polished. 



Having procured one of these crystals, properly 

 cleared and polished, place it over a sharp line in 

 the position represented in the annexed wood cut. 

 The eye being at R, the observer looking down 

 through the crystal will perceive two images of the 

 line, as m n, p q. In the same manner a spot or 

 luminous point will be doubly refracted by the 



crystal, and two images will be seen, as at e o, con- 

 versely ; if from the point R, where we have sup- 

 posed the eye to be, we cause a ray of light to fall 

 upon the crystal in the direction R r, it will, on 

 entering the crystal, be separated into two rays, r e 

 and r o, which two rays will, when they emerge from 

 or leave the rhomboid at e and o, proceed in the 

 direction e e', o o'', parallel to the incident ray R r. 

 Let us see what takes place when the incident ray 

 forms different angles with the plane of the crystal, 

 taking care to observe the positions of the two 

 refracted rays, r e and r o. When the ray R r is 

 perpendicular to the face of the crystal, or when 



the angle of incidence is 0, then the ray r o will 

 suffer no refraction, by the ordinary laws of optics, 

 but the other ray r e, does not obey this law, for it 

 will be found to be refracted at an angle of 6 12'. 

 In all other positions of the incident ray R r, the 

 refracted ray r o follows the common law of propor- 

 tionality to the sines, but not so with the ray r e. 

 The ray r o, therefore, is denominated the ordinary 

 ray, and the ray r e the extraordinary ray. 



In all doubly refracting substances there is at 

 least one line or plane along which no double 

 refraction takes place. According to the number 

 of these axes or planes of axes in a body, it is said to 

 be a crystal, or body of one, two, three, four, &c. 

 axes, or planes of axes of double refraction. When 

 the disappearance of double refraction arises from 

 the action of two opposing powers of double refrac- 

 tion, such axis is said to be a resultant or compen- 

 sation axis ; but when the disappearance of double 

 refraction does not arise from this cause, it is 

 said to be a real axis of double refraction. The 

 ray which is extraordinarily refracted, may either 

 take a direction towards or diverge from the axis 

 or plane of axis of the prism or crystal; in the 

 first case the axis is said to be positive, in the latter 

 case it is called a negative axis. 



According to Sir David Brewster, all doubly 

 refracting crystals that have the form of the rhom- 

 boid, the regular hexahedron, the square-based 

 octahedron, and the square-based right prism, 

 have all one axis of double refraction. Among 

 those which he examined, and of which he has 

 given an extensive list, we select a few of the 

 more prominent, prefixing the sign to those 

 whose axes are negative, and + to those whose 

 axes are positive. 



RHOMBS WITH AN OBTUSE SUMMIT. 



Carbonate of Lime. + Diop'asc 



Phosphate of lead. + Quartz. 



RHOMBS WITH AN ACUTE SUMMIT. 



Sapphire. Cinabar. 



REGULAR HEXAGONAL PRISM. 



Emerald. + Hydrate of magnesia. 



OCTAHEDRON WITH A SQUARE BASE. 



+ Zircon, + Prussiate of potash. 



RIGHT PRISM WITH A SQUARE BASK. 



+ Titanite. Subphosphate of potash. 



We will now attend a little farther to the pheno- 

 mena exhibited by the first single axis doubly re- 

 fracting crystal mentioned, i. e. Iceland spar. 

 Cut each of the faces of the crystal perpen- 

 dicular to the axis, and take care that they be 

 polished and plane ; it will be found that a ray of 

 light, which passes through the crystal in a direc- 

 tion parallel to its axis, does not suffer double re- 

 fraction, the ray continuing undivided, and the index 

 of refraction will be found to be 1-6543. Now if 

 the ray is made to pass through the crystal in any 

 other direction than that of parallelism to the axis, 

 the ray will be divided, or in other words, double 

 refraction will take place, and the ordinary and 

 extraordinary rays will have different indices of 

 refraction. It is remarkable, that whatever be the 

 direction of the incident ray, the index of refraction 

 of the ordinary ray remains the same, being invari- 

 ably 1-6543. When the incident ray falls upon the 

 crystal perpendicularly to any one of its faces, 

 (these faces being each at an angle of 44 36' 34" 

 to the axis) then must the plane of incidence pass 

 through the axis, and the index of refraction of the 

 ordinary ray still remaining the same as before 

 stated, the index of the extraordinary ray will be 

 found to be 1-572. Let the prism be now ground, 

 so that one of its faces shall be parallel to the 

 axis, and let the incident ray fall upon any part of 

 the prism perpendicular to the axis, then, although 

 the index of the ordinary ray be still the same, that 

 of the extraordinary ray will be found to be 1-4833. 



