OPTICAL INSTRUMENTS. 195 



through the spar will appear double, and the images will 

 be more separated from each other as the distance of the 

 object increases ; if the rhomb be turned about so as to 

 complete a revolution, the two images will have a regular 

 motion, so that one will fall twice on the prolongation of the 

 other. 



Fig. 711 is a drawing of a model representing a rhomb, 

 with a ray of ordinary light incident upon one of its natural 

 faces, and which, in passing through the crystal, is divided 

 into two rays, one of which is refracted according to the 

 ordinary way ; the other, not obeying the same law, but 

 being refracted in an extraordinary manner, is called the 

 extraordinary ray. Price, $2.50, $4.00, and $5.00. 



Nicholas Prism. Nichol's prism, or Nichol's eye-piece, 

 (Fig. 724, page 199), is a most valuable and convenient 

 polarizer. It is an oblique rhombic prism, whose terminal 

 planes form an angle of 68 with the adjoining obtuse lateral 

 edges. It is formed by joining two wedge-shaped pieces 

 of Iceland spar. It is a double refractor, but the two rays 

 are made to deviate so far that only one image is seen in its 

 usual position. Over the tourmaline this prism has the ad- 

 vantage of being perfectly free from color, but gives a much 

 more limited field of vision. It is usually mounted in a brass 

 cell with tube to hold by when rotated on its axis. 



Price, $5.00. 



Tourmaline Plates. The tourmaline is the most remarka- 

 ble among the crystallized minerals for possessing the pro- 

 perty of polarizing light transmitted through them ; for this 

 purpose the crystal is cut parallel to its axis, into thin plates 

 of a uniform thickness (about -$ of an inch) polished on 

 each side, and fixed between two plates of glass for pre- 

 servation, when it will possess this property in a remarkable 

 manner. Now, if we hold such a plate before the eye, and 

 look at the light of the sun, or the flame of a candle, a great 

 portion will be transmitted through the plate, which will 

 appear transparent, having only the color of the crystal, 

 which in specimens suited for these experiments is generally 

 brown or green ; but the light so transmitted will be polar- 

 ized light, and being analysed by a second plate which 

 may be done by looking through both at the same time 



