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The application of polarized light to the microscope 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 respecting 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 considered 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 designated "ether;" so that a beam of light may in section be represented by a circle, with a right-angled cross in it, thus ⨁.

To make what is meant by undulations more ntelligible, let the reader imagine a field of corn agitated by the wind—a succession of waves traverses it, each individual ear of cont 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 cars of com 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 undulations, 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 he placed over a black mark or spot on a piece of white paper, two spots are 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 eyepiece 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 we shall [Oct. 1, 1865. 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. First 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 then 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 character, and it is only in a certain position that it will pass through 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.

Nuthatches at the Window.—I was staying in Leicestershire in March, 1865, during the excessively cold weather and deep snow. I fed 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, their 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 hits 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 then at leisure.—F. Arnold.