Six Lectures on Light Part 13

'In every kind of magnitude there is a degree or sort to which our sense is proportioned, the perception and knowledge of which is of the greatest use to mankind. The same is the groundwork of philosophy; for, though all sorts and degrees are equally the object of philosophical speculation, yet it is from those which are proportioned to sense that a philosopher must set out in his inquiries, ascending or descending afterwards as his pursuits may require. He does well indeed to take his views from many points of sight, and supply the defects of sense by a well-regulated imagination; nor is he to be confined by any limit in s.p.a.ce or time; but, as his knowledge of Nature is founded on the observation of sensible things, he must begin with these, and must often return to them to examine his progress by them. Here is his secure hold: and as he sets out from thence, so if he likewise trace not often his steps backwards with caution, he will be in hazard of losing his way in the labyrinths of Nature.'--(_Maclaurin: An Account of Sir I.

Newton's Philosophical Discoveries. Written 1728; second edition_, 1750; pp. 18, 19.) ]

[Footnote 10: I do not wish to enc.u.mber the conception here with the details of the motion, but I may draw attention to the beautiful model of Prof. Lyman, wherein waves are shown to be produced by the _circular_ motion of the particles. This, as proved by the brothers Weber, is the real motion in the case of water-waves.]

[Footnote 11: Copied from Weber's _Wellenlehre_.]

[Footnote 12: See _Lectures on Sound_, 1st and 2nd ed., Lecture VII.; and 3rd ed., Chap. VIII. Longmans.]

[Footnote 13: _Boyle's Works_, Birch's edition, p. 675.]

[Footnote 14: Page 743.]

[Footnote 15: The beautiful plumes produced by water-crystallization have been successfully photographed by Professor Lockett.]

[Footnote 16: In a little volume ent.i.tled 'Forms of Water,' I have mentioned that cold iron floats upon molten iron. In company with my friend Sir William Armstrong, I had repeated opportunities of witnessing this fact in his works at Elswick, 1863. Faraday, I remember, spoke to me subsequently of the perfection of iron castings as probably due to the swelling of the metal on solidification. Beyond this, I have given the subject no special attention; and I know that many intelligent iron-founders doubt the fact of expansion. It is quite possible that the solid floats because it is not _wetted_ by the molten iron, its volume being virtually augmented by capillary repulsion. Certain flies walk freely upon water in virtue of an action of this kind. With bis.m.u.th, however, it is easy to burst iron bottles by the force of solidification.]

[Footnote 17: This beautiful law is usually thus expressed: _The index of refraction of any substance is the tangent of its polarizing angle_. With the aid of this law and an apparatus similar to that figured at page 15, we can readily determine the index of refraction of any liquid. The refracted and reflected beams being visible, they can readily be caused to inclose a right angle. The polarizing angle of the liquid may be thus found with the sharpest precision. It is then only necessary to seek out its natural tangent to obtain the index of refraction.]

[Footnote 18: Whewell.]

[Footnote 19: Removed from us since these words were written.]

[Footnote 20: The only essay known to me on the Undulatory Theory, from the pen of an American writer, is an excellent one by President Barnard, published in the Smithsonian Report for 1862.]

[Footnote 21: _Boyle's Works_, Birch's edition, vol. i. pp, 729 and 730.]

[Footnote 22: _Werke_, B. xxix. p. 24.]

[Footnote 23: Defined in Lecture I.]

[Footnote 24: This circ.u.mstance ought not to be lost sight of in the examination of compound spectra. Other similar instances might be cited.]

[Footnote 25: The dark band produced when the sodium is placed within the lamp was observed on the same occasion. Then was also observed for the first time the magnificent blue band of lithium which the Bunsen's flame fails to bring out.]

[Footnote 26: New York: for more than a decade no such weather had been experienced. The snow was so deep that the ordinary means of locomotion were for a time suspended.]

[Footnote 27: 'Il faut reconnaitre que parmi les peuples civilises de nos jours il en est pen chez qui les hautes sciences aient fait moins de progres qu'aux etats-Unis, ou qui aient fourni moins de grands artistes, de poetes ill.u.s.tres et de celebres ecrivains.' (_De la Democratie en Amerique_, etc. tome ii. p. 36.)]

[Footnote 28: At these points the two rectangular vibrations into which the original polarized ray is resolved by the plates of gypsum, act upon each other like the two rectangular impulses imparted to our pendulum in Lecture IV., one being given when the pendulum is at the limit of its swing. Vibration is thus converted into rotation.]

[Footnote 29: The millimeter is about 1/25th of an inch.]