580 
M. DES CLOIZEAUX OX AMBLYHOXITE AXE MOXTEBBASITE. 
71° in some specimens. By operating in air and in oil*' on two plates tolerably 
perpendicular to the plane of the optic axes and to their acute bisectors, I obtained at 
20° C. for the first plate : — 
2 E =S6 23 
2 It =55 18, whence 2 E = 86 28 
2 E =86 21 
2 11 = 55 8, whence 2 E = 86 19 
| Bed ray. 
| Yellow ray (soda). 
Bor the second plate : — 
2 E =70 54 
2 11 = 46 20, 
O / 
whence 2 E = 71 0 
Bed ray. 
2 E =70 32 
2 H = 45 59 30", whence 2 E = 70 32 
| Yellow ray. 
It is evident from these numbers that the actual dispersion of the axes is feeble, 
presenting g > vf. We come to the same conclusion by observing in air and in oil 
the fringes of the hyperbolas which traverse the two systems of coloured rings at an 
angle of 45° to the plane of polarization, and which present a reddish yellow on the 
inside and blue on the outside. A careful scrutiny further reveals that these fringes 
seem to have a slightly more decided colour in the one system than in the other, but 
in both the rings present to all appearance the same form. In the plane of polarization, 
the transverse bars of the central ring in each system are bordered by colours, the 
distribution of which in contrary directions indicates very distinctly a twisted dis- 
persion (“ dispersion tournante ”) : these colours appear furthermore to be slightly more 
definite round one of the bars than round the other ; but so slight a difference would 
not authorize us to admit the existence of a small amount of inclined dispersion, the 
combination of which with the twisted dispersion (“ dispersion tournante ”) would suffice 
to characterize a substance belonging to the triclinic system, unless the plates of Ambly- 
gonite from Montebras (formerly Montebrasite) possessed perfect homogeneity^. The 
* The oil I used was very viscous, aud had the indices M r = l*476; ra^l’478. 
.t In Montebrasite (formerly Amblygonite), State of Maine, one has, on the contrary, ^ <v on either side of 
the acute bisector ; so that this opposite character in the proper dispersion of the optic axes would suffice to 
distinguish Amblygonite from Montebrasite, since it forms a distinctive character of the highest importance in 
crystallized substances, and seems to be the most constant of all their properties as optically birefringent 
substances. In fact there is no example of a single well-defined species of a natural or artificial product 
presenting in different specimens optic axes with opposite dispersions, provided that these axes are situated in 
the same plane ; and this is true whatever may be the variations in their physical and chemical characters. 
It is only when the axes pass from one plane into a second perpendicular to it, as happens in orthose, 
heulandite, and other minerals, that any change takes place in the relative positions of the axes corresponding 
to the red rays with regard to the axes which correspond to the blue. 
J Under the same circumstances Montebrasite of Hebron and Montebras presents a horizontal dispersion in 
combination with a well-marked inclined dispersion. The phenomena to which those different kinds of 
dispersion give rise in the coloured rings are described and figured in my “ Memoire sur l’emploi du 
microscope polarisant,” in the ‘ Annales des Mines’ for 1864, tom. vi. 6th series. 
