Diffraction Experiments. 



125 



colours that we can see. The beautiful prismatic and other 

 colours, to be obtained by means of the polariscope, and various 

 bodies with which the microscopist is acquainted, are illustra- 

 tions of one mode of obtaining chromatic effects by the inter- 

 ference of rays of white light. They depend upon refraction, 

 and upon the path polarized light is able to take through 

 crystals and other substances capable of affecting its velocity. 

 Certain rays are bent out of their course, they are also more or less 

 dispersed* or resolved into prismatic rays, and these rays caused 

 by further refraction to meet in certain directions, and with 

 certain velocities, interfere with each other, and make the 

 gorgeous colour pictures in which the microscopist delights. 



If we can obtain interference of rays by any other means, 

 we shall produce dark bands, or coloured bands, according to 

 the circumstances of the case. 



The extreme red ray of the solar spectrum is produced 

 by undulations so numerous that, according to tables given 

 by Sir J.. Herschel, 37,640 of them only occupy one inch, and 

 458,000,000,000,000 occur in one second. These rays are, 

 however, large in the length of their undulations, and slow in 

 their movements compared with those of the extreme violet end 

 of the spectrum, in which 59,750 undulations only occupy one 

 inch, and take place at the rate of 727,000,000,000,000 in one 

 second of time.f 



Bearing in mind these considerations, let us now borrow 

 another diagram from Sir J. Herschel to show a simple case 

 of interference resulting from a wave of 

 light being partially obstructed by a nar- 

 row opaque body. O, Fig. 2, is a needle- 

 hole, through which a small sunbeam is 

 allowed to pass ; A B is a strip of card, 

 which, in the experiment performed by 

 Dr. Young, was -g^th of an inch wide; 

 E F, its shadow. Let the reader forget 

 for a moment the obstacle C D, which is 

 wanted for a subsequent experiment. A 

 B will then be the only obstacle in the 

 way of light getting from O to any point 

 between E and F. If we take the centre, 

 X, it will be plain that the light that 

 reaches : it will result from the spreading 

 portions of the wave passing tlje points 

 A and B, and as each extends laterally, 

 they will meet at X with equal velocities, 



FIG:2. 



* See paper on " Spectroscope Apparatus," No. 34. 



t These numbers will be slightly altered by the last estimation of the velocity 

 of light. * 



