REPORT ON OPTICS. 
319 
surfaces ;—the inclination of the resultant axes of crystals ha¬ 
ving double refraction, for different rays of the spectrum ;—the 
dimensions of the ellipse which regulates the polarization of 
metals and their alloys ;—the circularly polarizing forces of fluids 
and solutions ;—and the refractive and dispersive powers of or¬ 
dinary solid and fluid bodies, measured according to the method 
of Fraunhofer,—are some of the points to which we would call 
the attention of young and active observers. 
But important as these determinations would be in a scientific 
point of view, and particularly in the renovation of mineralogy, 
the application of the principles of double refraction to the 
examination of structures, is pregnant with a still higher 
interest. The chemist may perform the most dexterous analy¬ 
ses ;—the crystallographer may examine crystals by the nicest 
determination of their forms and cleavages ;—the anatomist and 
the botanist may direct the dissecting-knife, and use the micro¬ 
scope, with the most exquisite skill;—but there are still struc¬ 
tures in the mineral, the vegetable, and the animal kingdom, 
which defy all such methods of examination, and which will 
yield only to the magical analysis of polarized light. A body 
which is quite transparent to the eye, and which appears upon 
examination to be as monotonous in its structure as it is in its 
aspect, will yet exhibit under polarized light the most exquisite 
organization, and will display the result of new laws of combi¬ 
nation, which the imagination even could scarcely have con¬ 
ceived. Like the traveller who has visited an unknown land, 
polarized light emerges from bodies bearing with it the infor¬ 
mation it has acquired during its passage, and indicating the 
structures through which it has passed, when put to the ques¬ 
tion of optical analysis. As an example of the utility of this 
agent in exploring mineral, vegetable, and animal structures, I 
may refer to the extraordinary organization of Apophyllite and 
Analcime; to the symmetrical and figurate deposition of siliceous 
crystals in the epidermis of equisetaceous plants ; and to the 
wonderful variations of density in the crystalline lenses, and the 
integuments of the eyes of animals. 
One of the finest fields of optical inquiry, and one almost un¬ 
trodden, is that of the absorption of definite rays of the spec¬ 
trum by the specific action of the material atoms of those bodies 
through which light is transmitted, or from which it is reflected. 
The discovery of dark lines in the solar spectrum, is cer¬ 
tainly one of the finest which has been made in the present 
century, whether we view it in its theoretical bearings, or in its 
practical application to the construction of the achromatic tele¬ 
scope, and to the determination of all optical data which depend 
