224 



SCIENCE-GOSSIP. 



[Oct. 1, 1865. 



POLARIZED LIGHT. 



THE application of polarized light to the micro- 

 scope is of great value to the student, inasmuch 

 as by its aid the structure of many objects in the 

 animal and vegetable kingdoms, such as scales of 

 fish, hair, muscular structures, starch, granules, 

 sections of wood, &c., are more plainly made out, 

 and better seen; while in the mineral kingdom many 

 crystals may be distinguished from each other, though 

 having the same external form, by their action on 

 polarized light. 



To go deeply into all the beautiful theories respect- 

 ing the nature and properties of light, would form 

 an elaborate treatise, too voluminous for the pages 

 of Science Gossip ; but for our present purpose it 

 will suffice to state that light is now generally con- 

 sidered to consist of a series of undulations or 

 waves, taking place at right angles to each other 

 with inconceivable rapidity, in a fluid of extreme 

 tenuity which pervades all space, and has been de- 

 signated " ether ; " so that a beam of light may in 

 section be represented by a cii'de, with a right- 

 angled cross in it, thus 0. 



To make what is meant by undulations more 

 ntelligible, let the reader imagine a field of corn agi- 

 tated by the wind — a succession of waves traverses it, 

 each individual ear of corn oscillates on its stalk like 

 the end of an inverted pendulum moving backwards 

 and forwards a short distance, so that it is the 

 motion which travels, and not the ears of corn 

 through which it is conveyed ; so that as regards the 

 ultimate particles of the ethereal medium through 

 which the light is conveyed, we have motion without 

 progression of those particles. 



Certain bodies, known as double refractors, possess 

 the property of separating these undidations, and 

 Iceland spar (crystallized carbonate of lime) is the 

 one usually chosen for microscopic purposes, on 

 account of its freedom from colour. It is well known 

 that if an ordinary rhomb of Iceland spar be placed 

 over a black mark or spot on a piece of white paper, 

 two spots areO apparently visible. This phenomenon 

 is called double refraction ; but here the undulations 

 are not separated far enough for our purpose. 

 If we divide the rhomb into two equal portions in 

 a certain direction, then, having polished the cut 

 surfaces, and cemented them together with Canada 

 balsam, we shall find that, on looking through it at 

 the spot, only one image is visible, and we have 

 produced that very useful piece of apparatus called 

 the "Nicol prism." A pair of these adapted to the 

 microscope, one fitted beneath the stage, and called 

 the polarizer, and the other either above the eye- 

 piece or between that and the object-glass, and 

 called the analyser, for a reason which will soon be 

 apparent, and so adjusted that one or both prisms 

 may revolve on their respective axes, and wc shall 



have, with the addition of a plate of selenite mounted 

 as a slide, a most useful appendage to the microscope. 



Selenite is crystallized sulphate of lime, and has 

 been called by the miners, quarry glass. 



Now let us adapt our polarizing apparatus to the 

 microscope, and examine some of the properties of 

 the polarized light. Pirst of all we observe that in 

 a particular direction of the axes of the prisms to 

 each other, i. e., when they are parallel, light is freely 

 transmitted ; but if one of them is revolved so that 

 the axes of the two prisms are at right angles to 

 each other, the field will be dark. In passing through 

 the lower prism, the light has been altered in cha- 

 racter, and it is only in a certain position that it will 

 pass thi'ough the second; one of its undulations has 

 been, so to speak, bent aside out of its course, and 

 the other allowed to proceed straight on, until it 

 reaches the second prism, or analyser, which name 

 is now obvious ; for it analyses or tests the light, 

 showing by its passage or non-passage, in every 

 position of the axis of the prism, whether the light 

 is polarized or not. 



If we now interpose a plate of selenite between 

 the two prisms, we shall find the light is transmitted 

 through the whole combination in every position of 

 the axes, but is coloured according to the thickness 

 of the selenite, and the angle of the axes of the 

 prisms with regard to that of the selenite. 



A. J. PtOBERTS. 



Nuthatches at the Window.— I was staying 

 in Leicestershire in March, 1S65, during the exces- 

 sively cold weather and deep snow. I fed a great 

 many birds at the windows, and was anxious that 

 some nuthatches should come, as I have never 

 seen these birds in Westmoreland. I therefore 

 followed a plan I had read of, and tied some walnuts 

 to a string fastened to the window. I cracked the 

 walnuts so as to let the kernels be seen. In a short 

 time a pair of nuthatches came to the window, and, 

 having once discovered the good things there, theii 

 visits were incessant ; and they were not at all shy 

 of being watched. It was very amusing to see them 

 tugging away at the walnuts, and to watch their 

 evident impatience when they could not detach them 

 from the string ; knocking them fiercely against the 

 window-sill, twisting them backwards and forwards, 

 and hammering away at them with their strong 

 beaks, till they succeeded in carrying them off. 

 They seemed almost to prefer broken biscuit, I 

 suppose as being easier to get at. And they had 

 quite an unfair advantage over the other little birds, 

 for they held several bits at once in their long beaks, 

 packing them in one after another, and flew off with 

 them to the neighbouring deciduous cypress or 

 mulberry trees, to eat them at leisure. — F. Arnold. 



