5. TRICLINIC SYSTEM.

The cross appears through every plane inclined at a certain angle, when any one of the planes or corresponding edges of the crystal is placed horizontal or vertical on the engraved stage. Examples: Kyanite, albite, sulphate of copper.

ART. XLIV.-Stauroscopic and other Optical Experiments; by Prof. OGDEN N. ROOD.-Part I.

1. Stauroscopic Observations on Cooled Glasses.

IN the Stauroscopic observations of Prof. von Kobell plates of crystals with natural or artificial parallel sides were employed, and it is of course a matter of indifference through which part of the plate the polarized beam is transmitted, the phenomena observed being the same whether the centre or any of the edges be employed. The case is however different with plates of glass of a definite shape to which double refraction has been communicated by sudden cooling from a red heat or otherwise.

Having arranged a stauroscope with an open stage I submitted to examination pieces of glass of different figures to which double refraction had been thus communicated; the following are some of the simpler results obtained.

1. Through the centre of the equilateral triangle the cross arranges itself in three positions, at right angles to its three sides; the cross is of course not stationary when the triangular plate is revolved. The same is true when the polarized beam is transmitted through three spaces near the middle of its sides.

Through the angles of the triangle the cross is inclined to the above mentioned planes.

I found no spot in the triangle where the cross remained unaltered when the glass plate was revolved.

2. Through the centre of a circular plate of glass the cross remained tolerably unaltered by revolution, this position of the plate corresponds therefore to the basal plane of a crystal belonging to the dimetric system.

0

4.5

60°

Through the spaces 0, 0, 0, 0, the crosses are altered by revolution and arrange themselves in the positions of the dotted lines. Seen through the spaces marked with figures the crosses arranged themselves at these angles to the dotted lines.

1.

50°

30

2.

3. Through the centre of an elliptically shaped plate the cross was altered by revolution and arranged itself in the directions of the major and minor axes. The same is true for the spaces along the dotted lines.

The inclinations of the crosses to the axes of the ellipse in different portions of the plate will at once be seen by the annexed diagram, and also that the arrangement of the outer portions is essentially the same as in the circle, the main difference being in the occurrence of the oval-shaped spaces on either side of the foci.

4. Through the centre of the square the cross remains tolerably unchanged by revolution; near the middle of its sides the cross is altered by revolution and arranges itself at right angles to the sides.

60

40

65

70

70

309

40

3.

30

40

30

60

30

70

50

In the angles it was inclined to the sides of the square at angles of from 30° to 50°. In a perfectly evenly cooled plate it would probably arrange itself according to the diagonals. 5. Through the centre of a rhombicshaped plate the cross arranged itself parallel to only two sides of the rhomb; it remained always somewhat colored. (See under circular polarized light.) Through the spaces s, s, s, s, it arranged itself according to the diagonals.

S

S

6. Through the centre of a rectangle, whose length was four times its breadth, the cross arranges itself according to the diameters: in the angles the crosses are inclined at certain angles to the diameters.

When two pieces of this shape are laid one upon the other, parallel or at right angles, and viewed through their common centre, the cross still arranges itself parallel and at right angles to their sides.

When the plates were inclined 45° to each other no black cross was seen in any position; in its place a red or green nebulous cross appeared at all inclinations. (See below.)

45

4.

35

7. Through the centre of an octagonal shaped o plate the cross remained tolerably unaltered by revolution; in the spots marked 0 the cross was altered and arranged itself accordingly to the dotted lines; in the spaces marked with numbers the crosses made these angles with the dotted lines.

From these observations it will be seen that the crosses remain unaltered when viewed through the centres of regular polygons, as for example, the square, the octagon, and the circle, the latter being considered a regular polygon with an infinite number of sides; the centres of such polygons are therefore uniaxial and correspond to the basal plane of uniaxial crystals: the positions of these solids which I have marked 0 correspond with the pris matic faces of uniaxial crystals; the portions marked with numbers correspond in a certain sense with the planes of the quadratic pyramid. It is remarkable that we should have in portions of one plate a representation of the action of all the planes of a crystal.

The central portions of the ellipse and rectangle correspond to the basal plane of a crystal belonging to the rhombic system; the rhombic plate of glass had not been cooled with sufficient regularity to base a conclusion on its action though indications were observed which would place it along with the ellipse-at all events the spots marked s acted like the rhombic shaped ba sal plane of a rhombic prism.

The centre of the triangle corresponds in action to a plane of the quadratic pyramid.

2. Observations on Circular Polarization by means of Cooled Glasses.

(1.) The celebrated physicist, Dove, found when a cube of glass was suddenly cooled and afterwards placed in a polarizing apparatus and inclined 45° to the plane of primitive polarization, that the plane polarized beam was converted into circular polarized light by transmission through the corners of the square plate; this he proposed as an easy method of producing circularly polarized light.

I have found that when a beam of polarized light is transmitted through the centre of a rhombic shaped plate of glass (see above) which had been thus treated that the light was more or less circularly polarized whatever the position of the rhomb with regard to the plane of primitive polarization might be, though the experiment succeeded more strikingly when its position corresponds to that of Dove's cube. The polarization was righthanded, and the colors were as brilliant and followed each other with as much regularity as when seen through a quartz plate cut expressly for this purpose.

(2.) Through the centre and portions along the major diameter of an elliptically shaped piece of glass the light is also circularly polarized when either of the axes are inclined at angles of 45°, 135°, &c., to the plane of primitive polarization. The polarization was right-handed, and in both these cases a large beam of circular polarized light was obtained; by changing the angle of the

SECOND SERIES, VOL. XXVII, No. 81.-MAY, 1859.

plate with the plane of primitive polarization I easily obtained elliptically polarized light, the axes of the ellipse. having any desired relation to each other.

(3.) Through the centres and along the chief diameters of rectangular shaped pieces of glass, whose length was four times their breadth, the light was also circularly polarized when the inclination was 45° to the plane of primitive polarization. When two rectangles were crossed at an angle of 45° and placed as above, the light was more completely circularly polarized, it was found to be right-handed; upon reversing the inclination of the rectangles to each other the beam was turned to the left hand.

It will be seen therefore that by means of two similar rectangles of cooled glass, either right or left-handed circular polarization may be obtained at pleasure, an observation I believe which has never before been made.

(4.) The light in all the angles of the octagon was circularly polarized.

3. On the appearance presented by circularly polarizing crystals, &c. in the Stauroscope. The cross a means of detecting circular or elliptical polarization.

If a plate of quartz be cut at right angles to its axis and of such thickness that for example it gives the yellow tint when placed in the field of a polariscope, then when introduced into the stauroscope it will modify in a certain manner the cross and the colored rings.

(1.) No black cross will be seen in any position of the quartz plate; in its place a yellow cross appears which remains stationary and in its normal position when the quartz is revolved, and the white quadrants next to the calc spar cross are replaced with patches of red and blue color.

(2.) When the analyzing plate is revolved the yellow cross revolves with it, passing at the same time through all the prismatic tints, the rings not being greatly changed. If the plate is thicker or thinner than the above mentioned, the initial cross will merely in the first instance be differently tinted, but the color will in every case be the same with that which the whole plate would have if placed in the darkened field of a polariscope; the rest of the phenomenon remains the same. The circular polarized light obtained through cooled glass acted in exactly the same manner. I have found this colored cross, stationary and revolving, an excellent means of detecting circular polarized light when it would otherwise have been overlooked, that is, when such a brilliantly colored cross is seen which revolves with the analyzer through the whole circle, changing always in tint but never becoming black, it is a proof that the light is circularly polarized.

When the polarization is elliptical a colored cross it is true. will be seen and it will revolve with the analyzer and change in tint, but at angles of 180° it becomes normal and nearly black.

Of course light occurs in every state of polarization, passing from elliptical through circular to plane, according to the relative dimensions of the axes of the ellipse, but after some practice it is possible by means of the cross to judge of these dimensions approximately.

In examining this matter, besides using plates of quartz, &c., I constructed an apparatus for producing plane, circular, or elliptical polarization by means of the inclinations to the plane of primitive polarization given to a lamina of mica of a certain thickness, the state of polarization of the light from the mica was examined with a doubly refracting rhomb of cale spar before observing the cross.

Troy University, Dec. 25th, 1858.

ART. XLV.-On Boltonite; by Prof. GEO. J. BRUSH.

THE identity of this mineral with chrysolite was pointed out by Professor J. Lawrence Smith in this Journal, vol. xviii, p. 372 (November, 1854). Previously to this Dr. Kenngott* endeavored to show from analyses made by v. Hauer that boltonite was a distinct species. Subsequently + Kenngott calls in question Smith's conclusions: from v. Hauer's analyses he draws the formula RS, and insists that this, together with the low hardness (55) of the mineral, shows that boltonite is not chrysolite. In Dr. Smith's examination, the mineral was as carefully selected from the gangue as possible and then freed from adhering carbonates by repeated treatment with dilute hydrochloric acid; afterwards great care was taken in choosing for analysis the purest of the fragments thus treated. Three analyses made upon specimens so selected gave:

which excepting the slight amount of loss by heat (not necessarily water) indicate a perfect correspondence with chrysolite.

Von Hauer did not find it practicable to separate the boltonite from the gangue, but analyzed a specimen with the accompanying gangue. In his first analysis the mineral was decomposed by fusion with carbonate of soda. His results gave

*Mineralogische Notizen, Zwölfte Folge (March, 1854).

+ Uebersicht der Resultate mineralogischer Forschungen im Jahre, 1854. Leipzig, 1856.