CHAPTER XI.

POLARISING APPARATUS.

The Nicol Prism-Prazmowski's Modification-Large and Small Analysers-Nicol Prism Polariscope-Foucault's Prism-Care of Prisms--Glass Piles-The ordinary Lantern Polariscope-Simple Apparatus for Private Study-Norremberg's Doubler.

127. The Nicol Prism.-The tourmaline is an admirably simple polariser or analyser; and it gives a "field" of any angle, which makes it very convenient for "eye" work: it is however too small for large polarisers, and its colour is a serious drawback.' The polarisation by Iceland spar is also perfect; and as this spar is as clear as glass, if we can abolish one of the rays we shall get a colourless and perfectly polarised beam. This was effected by Nicol in the prism which bears his name. Reflecting that the two indices of refraction varied considerably (they are 148 and 1.65), and that Canada balsam, so much used for cementing lenses, was intermediate between these, he cut a crystal of spar thrice as long as its diameter through A B, Fig. 149, and cemented the

1 The largest tourmaline known is in the possession of Dr. Spottiswoode, and is over two inches square. Strange to say, its colour and polarising properties are both remarkably good. It is difficult to get really good plates, tolerably free from colour, more than 2 inch square, and such a plate is worth several guineas.

sections by a layer of balsam. Then a ray C D on entering the spar is doubly refracted, one half more so than the other. The most refracted half finds in the layer of balsam a less refracting medium; and obviously, if the angle is oblique enough for the respective indices, must be "totally reflected" to one side, and is thus got rid of. The other ray finds in the balsam a denser medium, and therefore passes through.

Nicol prisms are sometimes made with the end faces cut square to the axis. These need more spar, but we avoid part of the reflection from the inclined faces. Prazmowski further improved them by making the joint with linseed oil. There is difficulty in baking this dry, about which there seems some secret: but as the "index" of the oil is nearer

than balsam to the lower one of the spar, an addition to the angular field is gained.

The "Nicol prism" is the most perfect piece of apparatus we possess, owing to the absolute polarisation of all colours alike, and its freedom from colour. One about 2 inches long will cost from 30s. to 50s., according to where it is purchased, and should be fitted in a tube which fits and rotates on the nozzle of the objective, (N, Fig. 1) to serve as an analyser. A large Nicol also makes the most perfect "polariser"; and the cost, which is many pounds, is its only objection. Large spar is scarce, and therefore dear; and a prism of 2 inches aperture requires a rhomb more than 6 inches long. If such a prism can be afforded, it should be mounted in front of the lantern flange,

so that the full parallel beam passes through it, and so as to be capable of rotation. With such a polariser, we can turn the lantern direct to the screen, which is a great advantage; as also is the power of rotating the full beam as to its plane of polarisation. But, on the other hand, a large Nicol for analyser, as used by our chief lecturers, I consider a mistake in almost every way.' We must manifestly focus on the screen all the slides we employ, by a lens which will converge and cross all the rays. Now referring to Fig. 150, it will be seen that unless the lens be of long focus, the small Nicol, properly adjusted at the crossing point, allows every ray to pass through it which can possibly get through the larger one, however large it be; and thus a small Nicol so

FIG. 150.-Large and Small Analysers.

adjusted actually saves absorption of light through a great length of spar. Moreover, we can adjust a small Nicol in this way precisely, and focus with precision by the rack and pinion of our objective; whereas with a monster Nicol in front, both the object and the focusing lens have, as usually mounted, to be coarsely adjusted by hand, and much light is scattered about the room. Again, to ensure getting most of the rays through a long Nicol, a lens of long focus has to

1 A little qualification is necessary, because in some investigations it is desirable to carry a beam of parallel light through polariser, object, and analyser, to a prism or other apparatus; and for such experiments a large analyser has obvious advantages. Such experiments are however few, belong chiefly to scientific investigation, and are not suitable for projection.

be employed; which either necessitates a long screen distance, or gives a smaller image, and will hardly show any rings in crystals without additional and special apparatus. For these reasons, and after seeing experiments performed with large Nicols repeatedly, I adhere to an opinion based upon experience, that such a lantern-front as described in Fig. 1, with a small Nicol adjusted at the right point, gives more light, a better disc, greater facility in manipulation, and superior effects in almost every way.'

128. Nicol Prism Polariscope.-It may be well to describe a projecting polariscope (which is of course capable of private or eye-work also, by the light of a candle or that from a window,) made on this principle; and especially as I have never seen an instrument which, taken all round, was capable of such a variety of work with so little trouble and such rapid facility in manipulation, especially in the dark; a column of fluid, or the convergent arrangement for wideangled crystals (see § 167) being added or withdrawn in twenty seconds. The adjustments have all been most carefully planned, several details of value being suggested from their own experience by Messrs. Darker, who constructed the whole with much care and skill from a Nicol of barely two inches

It is scarcely necessary to state that nothing is further from the purpose of these pages than any depreciation of apparatus which is the admiration-almost the envy-of other than the happy possessors. The first pair of large Nicols made for Dr. Spottiswoode were of distinct scientific value, as being the precursors of this kind of apparatus, never before made on such a scale (they were about 2 inches clear aperture, and larger ones were afterwards made for the same gentleman, over 3 inches). And the larger a polarising Nicol can be secured or afforded, the better. But I certainly do desire to make clear to science-teachers and others, how very nearly all the experiments capable of projection by these magnificent instruments can be equalled on the screen at a far less prohibitory cost, not altogether beyond the means of some who might otherwise never dream of possessing such a class of apparatus.

aperture, but of great purity, made for me by Mr. Ahrens. The drawing is on a scale of three inches to the foot.

B (Figs. 151 and 152) is a base-board, barely twenty-four inches long, and six inches wide, in which mahogany slides, H (Fig. 151), are secured by screws, L (Fig. 152), which tighten the dovetailed or rebated slides. The screws bear upon slips of brass fixed to the slide-bases, H, as usual. On

these bases are screwed brass standards which carry separately the polarising Nicol, PN, the stage and focal power, S, F, and convergent lens system when required (Fig. 152).

The large Nicol, P N, is mounted in an inner tube, fitted in a larger tube-partly for appearance and partly to accommodate a rather larger Nicol should such ever be acquired. To save useless weight and space, its corners are cut off in a hexagonal form. The outer tube carries at the end next the lantern a flange, D (Figs. 151 and 152), divided into forty-five degrees, and at its middle four spokes, by which