246 USE OF THE MICROSCOPE. 



adopted, and has received the name of the undulatory theory. 

 Light, as is well known, is made- up of three colours, each 

 colour being produced by a wave or undulation of a particular 

 length; if all meet together in the same state of vibration, 

 white light is formed, but if they meet under other conditions, 

 darkness or colour may result from the waves interfering with 

 each other. If the difference of length between two waves be 

 an even number of half undulations, the two will coincide and 

 produce a colour equal in intensity to the two combined ; but 

 if the difference be an odd number, darkness will result. Let 

 us now apply this principle to the selenite between the two 

 prisms, and let us suppose the prisms to be arranged so that 

 no hght may be seen when the eye is applied to the eye-piece ; 

 the polarized beam, therefore, from the first prism not being 

 able to pass through the analyzing prism, a plate of a double 

 refracting crystal is introduced; this has the property of 

 dividing the polarized beam into two rays, which are polarized at 

 right angles to each other ; but at angles of 45" to the original 

 ray, these falling on the analyzing prism, and being inclined 

 somewhat nearer to its crystallographical axis than the polar- 

 ized beam originally was, some of its vibrations will pass 

 through, and colour will be produced by interferences in the 

 undulations ; and if, in the first case, the colour be red, then, 

 as the prism is turned, it will become green. The red and 

 green colours are, as has been before stated, complementary, 

 or the one is what the other wants to form white light ; if a 

 double refracting crystal of Iceland spar be used instead of the 

 Nicol's prism, it can be shown that where the red and green 

 are super-posed, white light results. But as this subject will 

 be best understood by reference to a diagram, the author has 

 selected the following from the second edition of the valuable 

 Introduction to the Study of Polarized Light, lately published 

 by his friend, Mr. Woodward. In order to render the 

 diagram more intelligible, it may be as well here to state 

 that ordinary light is represented by a cross, which denotes 

 that its vibrations are in planes at right angles to each other, 

 whereas, when one set of such vibrations only is shown, the 

 light is said to be polarized. Mr. Woodward's description of 

 the production of colour by polarized light being in itself 



