C1UCULAK POLARISATION.] 



UNUULAl'ORY FORCES. LIGHT. 



85 



extraneous rays. These tubes mul be of various lengths, 

 to suit the different liquids which Fig. so. 



may be employed ; and they are 

 held upright between the eye- 

 piece and the polariser, by means 

 of a ring, which is again at- 

 tached, by the screw, c, to the 

 ring-stand : d is an ordinary 

 polarising plate, attached to the 

 stand by the screw, e ; and /is the 

 eye of the observer. Wo need 

 scarcely state, that the analyser 

 must so rest in its socket, as to be 

 readily turned round through the 

 circular ring, as by means of its 

 rotation over the tube, 6, the phe- 

 nomena are observed. Incomplete 

 instruments, a rests in a graduated 

 frame, which is essential for ac- 

 curate experiments, because the 

 angle of rotation varies for each 

 liquid examined; and their dif- 

 ferences, with respect to po- 

 ll light, are so pointed out. The bottom of the 

 titl>o, 6, should be as flat as possible, so as to prevent 

 the loss of light on account of its external curvature. 

 With such an apparatus, which is easily constructed, 

 indent will have a wide field for research. He will 

 be enabled to detect physical characteristics in bodies 

 i no other means can point out ; his ideas in 

 nee to the constitution of such will be extended ; 

 :n merely examining the external peculiars. 



i < substances presented to him, he will be able, 

 as it were, to acquaint himself with their internal consti- 



in a manner as astonishing as it is instructive. 

 \V.! m iv le '< shortly point out the essential difference 

 which exists between plane and circular polarisation. 

 In the former, the chief effects are witnessed when the 

 planes of observation are at right angles to each other, 

 rular polarisation these angles vary, and depend on 

 circumstances, which, in their turn, arise directly from 

 the peculiar or individual constitution of the body under 

 examination. Wo should remark, that the licst plan of 

 observing the phenomena of circular polarisation, is 

 that of using homogeneous or single-coloured light;* 

 and this is easily managed by interposing a plate of red 

 glass between the eye and the analysing prism. This 

 prevents a variety of disturbing causes; and it also 

 ensures the disappearance of the extraordinary ray. t 



We may mention, that some little difficulty occurs to 

 the beginner in observing the direction of rotation; 

 because, as we have just stated, different liquids require 

 greater lengths before the phenomenon in exhibited. 

 The following, however, we may adduce respectively, as 

 instances in which the effects may be observed ; and, as 

 they have all been tried in an equal length of tube 

 namely, in one of about six inches they will afford a 

 starting-point, from which the. student may commence 

 his progress. We have also given the angle, as indicated 

 by the index on the analyser, which completes the arc 

 of rotation for each. 



Patterning Left-handed PoUtritation. 



Oil of turpentine 45 



Naphtha 124<y 



Fattening Right-handed Polarisation. 



Oil of bergamot 29 



Weak solution of cane sugar in water . 23 5' 

 of milk . 10 3' 



It will thtu be observed, that the angle of the arc of 

 rotation, and its direction, become definite physical cha- 

 racteristics of the various bodies in solution to which we 

 may apply the test of polarised light. These researches 

 have been of the greatest advantage to the chemist a* a 

 i of analysis ; and, as such, we cannot too highly 

 value the results which have been obtained. We must, 

 how entering fully into the < n of 



these interesting facts, to our section on < 'h< -niUiry, in 

 Src mil, p. M. 1 <"tlt, p. 79. 



which we shall more completely show their special 

 applications. 



If our space, and the character of our work permitted, 

 we might examine a great variety of subjects inciden- 

 tally connected with, or elucidating, the phenomena of 

 polarisation. Some of these we shall have to consider 

 when we deal with what we may term the Magnetic 

 Relations of Light. Others will come under our notice, 

 under the sections of Mineralogy and Crystallography ; 

 and, as such, we shall have a better opportunity of 

 dealing individually with special cases, than we have at 

 the present time, whilst endeavouring to lay down and 

 illustrate laws which have a general rather than a par- 

 ticular application. 



Before concluding, however, our remarks on the 

 Polarisation of Light, we nmy refer our readers again to 

 the analogy which exists between the same phenomena 

 in connexion with heat. The observations we made 

 previously,! will have prepared the student for the more 

 complex phenomena of luminous over calorific rays ; and 

 what we have just finished discussing, will, in a similar 

 manner, elucidate and extend the doctrines of calorific 

 refraction. We may also add, that the analogies of the 

 two forces of heat and light, thus seem, at each step, to 

 converge together ; and, like the vanishing points in a 

 landscape, their differences seem to grow less in propor- 

 tion as our view of them becomes more extended and 

 comprehensive. 



As wo here must end our examination into the laws 

 of light, we may remind the student, that many of the 

 facts we have been investigating will again require our 

 attention when we inquire into the phenomena produced 

 by electricity, magnetism, dia-magnetism, and sou ml. 

 Each of the forces ranked under the term " undulatory," 

 has necessarily a mutual connection. In fact, they 

 sometimes seem as if one force, modified by special cir- 

 cumstances, or the affections of matter, would satisfy 

 the conditions implied in what we now recognise as so 

 many separate and independent agents. 



have already seen how far heat and light art. 

 similarly subject to the same results when undergoing 

 ion, refraction, and polarisation. We shall find 

 that sound is partly subject to the same influences; and 

 that electricity and magnetism have a connection with 

 both the agents to which we have already directed 

 attention. 



The attentive reader and student will reap great 

 advantage in following, as closely as possible, the various 

 relations in which these questions stand to each other. 

 The nature of our plan forbids our entering into minute 

 detail ; but the student can, at his leisure, ponder over 

 and collect these analogies, and thereby ho will find 

 that each subject and section, as wo proceed, will form a 

 kind of exposition to others. One of the gn. 

 benefits, in an intellectual point of view, which attend* 

 the study of experimental science, is its tendency to 

 expand our powers of generalisation. By giving us a 

 broad and expansive view of creation in general, wo are 

 insensibly raised above that narrow individuality of 

 character which is so natural to us all. Whilst we 

 mentally roam through the vast expanse of truth, we 

 become enlarged in our ideas, chastened in our imagiua- 

 tion. and strengthened in our reasoning powers. 



Having thus considered the chief laws of light, we 

 now proceed to some of its applications, in which we 

 shall find the theories already explained put more or less 

 into practical purposes. In these branches of our sub- 

 ject, the reader will not fail to notice how much inoro 

 rapidly improvements in matters of daily life take place, 

 when under the fostering guide of sound science, than 

 under the uncertain ,/;, -la of crude and ancient empiri- 

 cism. Yet we find that, although much progress has 

 been made in many manufactures by men utterly ignorant 

 of scientific laws still such persons, generally speaking, 

 gradually acquire an amount of information which is a 

 partial substitute for sounder acquirements. Of this fact, 

 many instances might be quoted ; such as the. production 

 of the Tynan purple, the staining of ancient glaH and 



1 Sec Mitt, p. 29. 



