236 USE OF 'J'HE MICROSCOPE. 



for making sketches of landscapes : this being fastened to the 

 table, D, by the screw, b, and the object, M N, set up in front 

 of it, an accurate outline on a larger scale, M' N', can then be 

 made on the floor, as in the preceding method ; the pencil, P, 

 with a long handle, F, being held by the hand, H, the artist 

 standing either in front or on one side of the camera, and 

 applying his eye to it as at E, The size of the picture will, 

 like all others made by the camera, depend on the relative dis- 

 tance between the object and the paper. It will, however, be 

 found in practice, that about four feet wiU be the utmost limit 

 of the space between c and P to allow of the pencil being 

 used with any advantage. In this case a lens must be placed 

 at c, before the prism, suited to make the object appear just 

 as far as the floor. 



CHAPTER VI. 



ON THE POLAKIZATION OF LIGHT. 



Origin of the Term. — " If," says Sir D. Brewster,* " we 

 transmit a beam of the sun's light through a circular aperture 

 into a dark room, and if we reflect it from any crystallized or 

 uncrystallized body, or transmit it through a thin plate of 

 either of them, it will be reflected and transmitted in the very 

 same manner, and with the same intensity, whether the surface 

 of the body is held above or below the beam, or on the right 

 side or left, or on any other side of it, provided that in all 

 these cases it falls upon the surface in the same manner, or, 

 what amounts to the same thing, the beam of solar light has 

 the same properties on all its sides, and this is true of light 

 emitted from candles or any luminous bodies, and all such 

 light is called common light." But if the same light be allowed 

 either to fall upon a rhomb of Iceland spar, or upon a plate of 



* Treatise on Optics, page 157. 



