GONIOMETER. 251 



polarized light. Some of the most delicate of the elementary- 

 tissues of animals, such as the tubes of nerves, the ultimate 

 fibriUaj of muscle, &c., are amongst some of the most striking 

 subjects that may be studied with advantage under this 

 method of illumination. It would be impossible, in a work 

 like the present, to give a long list of objects that require the 

 aid of polarized light for their exhibition; every structure 

 that the microscopist is investigating should be examined by 

 this light, as well as by that either transmitted or reflected; 

 objects mounted in Canada balsam, that are far too delicate to 

 exhibit any structure under ordinary illumination, wiU often 

 be well seen under polarized light; its uses, therefore, are 

 manifold. Those who would wish to enter scientifically into 

 this subject, should consult the works of Biot, Brewster, 

 Herschel, and the excellent Lectures of Dr. Pereira; the 

 Familiar Introduction to the Study of Polarized Light, by Mr. 

 Woodward, may also be studied with very great advantage. 

 The object of the author in the present chapter has been to 

 show the nature of the apparatus employed, how it is adapted 

 to the microscope, and the method of using it, together with 

 as short an explanation as possible of some of its most important 

 principles; the mode of preparing such objects as wUl best 

 exemplify these principles will be fuUy described in a subse- 

 quent chapter. 



CHAPTEE VIL 



GONIOMETER. 



Foe the purpose of measuring the angles of crystals, whether 

 microscopic or others, a very beautiful instrument has been 

 invented by Dr. Leeson, and has been termed by him, a 

 Double Refracting Goniometer. A full description of it has 

 been given in the Proceedings of the Chemical Society, 

 Part XXIII., of which the following Is a transcript : — 

 " The goniometer, invented by the author with a view to 



