J. V. HAEFOED — POLARISATIOJf OF LIGHT. 157 



An exceedingly interesting department of the subject must now 

 be taken into consideration, that is, coloured polarisation. The 

 beautiful phenomena, usually displayed by means of the microscope 

 with a polariscope attached, are amongst the most interesting as 

 well as the most gorgeous revelations of natural science. It will 

 now be our object to illustrate and explain the principles on which 

 they depend The subject of colour in general would alone furnish 

 matter for long and interesting research, but on this occasion we 

 are restricted to so much of the subject as is concerned with polar- 

 isation only. All colour is caused by light, and thus so many 

 things are common to the whole subject that we must be allowed 

 some extent of comprehension and collateral remark in treating 

 even a subdivision. The colours exhibited by objects examined by 

 the aid of polarised light are caused by what is called theoretically, 

 " interference," i.e., interference of waves of light. Thus a theory of 

 waves is involved, or the so-called undulatory theory. Now what- 

 ever may be the exact character or form of the so-called waves of 

 light, there is clearly, from the result of observation and experi- 

 ment, a close, exact analogy with the theory and action of waves 

 in general. If nothing else is proved, this characteristic at least is 

 undoubted, viz. a regular recurrence of inteiwals, or periodicity, in 

 the action of light ; and it may also be affirmed that this periodicity 

 is capable of being made the basis of exact calculation on the 

 mechanical principles which govern the theory of wave action 

 generally. 



The principal points may be here enumerated, and the undulatory 

 theory of light will be found in accordance, if not in actual form, 

 at least, in strict analogy. 1st. "Wave motion commences at a 

 central point of disturbance, and is propagated in all directions 

 around, by regular equal-timed waves. Fig. 7 will explain this 

 principle, both in reference to water and air, the latter producing 

 sound. 2nd. The wave is formed by the vibrating movement of 

 particles of the medium to and fro, — in water, transverse to the 

 line of propagation of the wave, — in air, longitudinally ; each half 

 wave being contrary to the adjoining half in respect to the state 

 or phase of its vibrations. 3rd. The wave, or rather the wave 

 form, travels, and not the particles of the medium, which merely 

 oscillate to and fro ; the front of the wave being formed of particles 

 in the upward phase, the back of the wave of particles in the 

 downward phase. 4th. When two waves encounter each other, 

 the result for water or other liquid is an increase of height or 

 intensity of form at those points of intersection where summit and 

 summit or hollow and hollow coincide, and a calm or mutual 

 extinction where summit and hollow intersect. At all intermediate 

 points the interference is proportionate to the relative distances 

 from the points of complete intersection. (Figs. 8, 9, 10, 11.) In 

 air the effect is analogous, taking condensation and rarefaction as 

 analogous to summit and hollow, and intensity and calm as analo- 

 gous to sound and silence. In all these particulars the undulatory 

 theory of light is in strict analogy. 1st. The equal and equal- 



