4 LECTURES TO SCIENCE TE AGREES. 



ray of which it is composed ; in other words, the effect belongs 

 to a general direction running through the whole, or to a 

 direction determined by a plane, e.g., that of the polariser. 

 Moreover, since, as described before, the beams reflected and 

 refracted by the analyser are respectively extinguished by the 

 analyser in two positions at right angles to one another, the 

 two beams are said to be polarised in two planes at right 

 angles, or to have their planes of polarisation at right angles 

 to one another. This, then, is one test of polarised light. 

 If we proceed in the same manner to examine the beam trans- 

 mitted by the polariser, similar effects will be noticed, except 

 that the positions for extinction will be reversed. The extinc- 

 tion described is, however, in fact, only partial, not complete ; 

 but it will be enhanced or rendered more complete if, instead 

 of using a single plate of glass, we use a number of plates 

 placed close together. The reason is this, that the effect is 

 produced in the passage of the light through the glass, and 

 the effect produced by one plate is carried on to a greater 

 degree by a second, and so on until by the use of a sufficient 

 number a state of polarisation as nearly complete as we 

 desire may be obtained. With a dozen plates of thin micro- 

 scopic glass excellent effects are produced ; but five or six 

 plates of ordinary thin glass, say 1J inch long and f inch 

 broad, held obliquely at the angle before described, will do 

 very well, and is altogether preferable to a single plate. 

 From the fact that, when the plates are properly placed, both 

 the reflected and the transmitted rays are polarised, there 

 follows the important principle that, whether we use reflected 

 or transmitted light, we may at pleasure interchange the 

 polariser and analyser j in short, that any instrument which 

 will serve for one purpose will serve for the other. The 

 bundle of glass will therefore furnish you with a beam of 

 reflected and a beam of transmitted light, both polarised ; 

 but since the transmitted beam preserves its direction when 

 the bundle is turned round, while the reflected beam does 

 not, it is generally more convenient to use the former. 



It was on account of the fact that a beam of light, when acted 

 upon in a certain direction across its line of progress, is under 

 certain conditions capable of being extinguished by processes 

 which do not ordinarily extinguish light, that a beam t of light 

 when so acted on was said by Sir Isaac Newton to have sides. 

 And inasmuch as this effect had reference to direction, it was 



