88 



of fluids and solutions ; and the refractive and dispersive powers of 

 ordinary solid and fluid bodies, measured according to the method of 

 Fraunhofer, — are some of the points to which we would call the at- 

 tention of young and active observers." 



EXPLANATION OF PLATE XII. 



Fig. 1 represents a bundle of plates of crown glass, in which a ray of common light, 

 having two or more planes of vibration, is reflected at an angle of 56° from 

 the perpendicular, the portion which is reflected having one plane of vi- 

 bration, and that which is transmitted vibrating in a plane at right angles 

 to it. 



Fig. 2. A plate of tourmaline : the light which emerges after transmission is found to 

 consist of polarized light, vibrating in a plane corresponding to the axis of 

 the crystal. 



Fig. 3. A Nicol's prism : a ray of light, on entering the prism, is divided into two 

 pencils, one of which is transmitted, and the other is so refracted, by means 

 of the glass, that it is thrown out of the field. 



In these figures, the arrows indicate the direction of the ray of light ; the 

 circles with crossed lines represent common light ; and those with only one 

 line polarized light, with its plane of vibration. 



Fig. 4 represents the complete separation of the images by the double-refractor, and 

 the effect of passing the larger orifices of the brass plate along the stage. 



Fig. 5 shows the effect of adding a Nicol's prism and plate of selenite, with the de - 

 composition and recomposition of white light. 



Fig. 6 gives the appearance of revolving the double-refractor, and the alternate cutting 

 off of each image, at every quarter of a revolution (as in Exp. 2). 



Fig. 7. The same as before, with the addition of a plate of selenite. 



Fig. 8. The effects produced by two double-refractors, in separating the images to 

 twice the distance from each other, compared with those in fig. 4, and the 

 appearances presented at every quarter revolution of the second crystal. 



Fig. 9. The same as above, with the interposition of the plate of selenite. 



Fig. 10. The most suitable position for the Nicol's prism in investigations where a 

 plate of selenite is employed. 



Fig. 11. The proper position of the selenite, in relation to the polarizer, to produce 

 the best effects. 



Fig. 12. The Nicol's prism, turned about 45° from its former position, which should 

 be carefully avoided in using the selenite plate. 



Figs. 13, 14 and 15. The brass mountings of the double-refractors and plate of 

 selenite. The crystals and selenite should be adjusted to the relative posi- 

 tions indicated in figs. 10 and 11, and the slits in the mounting placed in 

 a line, to produce the appearances in fig. 9. 



