83 



XV. — On the Application of Polarized Light in Microscopic Investi- 

 gations. By M. S. Legg, Esq. 



(Read December 9, 1846). 



It has been truly and eloquently observed by Sir David Brewster, 

 that " the application of the principles of double refraction to the ex- 

 amination of structures is pregnant with a very high interest. The 

 chemist may perform the most dexterous analyses; 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 microscope with the most exquisite skill ; but there 

 are still structures in the mineral, the vegetable, and the animal king- 

 doms, 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 in its aspect, will yet exhibit 

 under polarized light the most exquisite organization, and will display 

 the result of new laws of combination, which the imagination even 

 could scarcely have conceived. Like the traveller who has visited an 

 unknown land, polarized light emerges from bodies bearing with it 

 the information.it has acquired during its passage, and indicating the 

 structures through which it has passed, when put to the question of 

 optical analysis. As an example of the utility of this agent in ex- 

 ploring mineral, vegetable, and animal structures, I may refer to the 

 extraordinary organization of Apophyllite and Analcime ; the symme- 

 trical and figurate depositions 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." * 



In order, therefore, to facilitate the application of this principle, 

 and to render the subject interesting to those who may be disposed to 

 adopt it in microscopic investigation, or who may already have the 

 necessary apparatus fitted to their instruments, as well as for the infor- 

 mation of those whose avocations may have debarred them from the 

 study of this branch of natural philosophy, it is proposed to describe 

 a series of apparatus, and to detail some experiments which, although 

 well known to a few, are not generally understood by mere microscopic 



* Report on Optics, Brit. Ass. 1832. 



