\VITH THE MICROSCOPE. 175 



simple. Thin plates or large segments may occur, the material 

 of which is not in the same crystalline position as the rest, but 

 inclined at different angles, according to the laws of twin crystals ; 

 and in some cases this gives rise to very remarkable characters, seen 

 to great advantage in some of the constituents of meteorites. On 

 the contrary all perfect crystalline planes may be absent, and yet the 

 ultimate structure may be that of a simple and perfect crystal, as 

 shown by cleavage, or by the action of polarized light ; and there- 

 fore it becomes necessary to understand what terms should be 

 employed to express these facts. Shall we use the term a crystal to 

 signify a body bounded by definite planes, which may have a very 

 composite internal structure, or to signify a body of perfectly simple 

 and uniform molecular constitution, which does not happen to have 

 perfect bounding planes? In studying the structure of rocks it 

 appears to me far better to use the term a crystal to signify the sim- 

 plicity of ultimate molecular constitution, and to express the character 

 of the external form by saying whether it is bounded by crystalline 

 planes or by irregular surfaces, independent of the crystalline 

 structure. 



If we use this phraseology we may say that each detached plate 

 of the echinodermata is one crystal, being as if it were made out of 

 one simple crystal of calcite cut into the form and hollowed out into 

 all the complicated structure characteristic of that kind of shell ; and 

 as an example of the very opposite fact, I may refer to certain 

 crystals of native phosphate of lime, which have the external planes 

 characteristic of that mineral, and yet are made up of a vast number 

 of much smaller crystals, in no way related to the external form, and 

 not bounded by crystalline planes. Such distinctions are amongst 

 the most important in studying the microscopical structure of rocks, 

 and from such facts the physical history of a rock may be deduced. 



266. Of the use of Polarised Light. In applying these principles 

 it is requisite to understand some of the leading facts connected with 

 polarised light. It must not be used merely to show structure, or, 

 as is too often the case, merely to show pretty colours, but as the 

 most searching means of learning the nature and molecular constitu- 

 tions of the substances under examination. The action of crystals 

 on polarised light as applied to the microscope is due to their double 

 refraction, which depolarises the polarised beam, and gives rise to 

 colours by interference, if the crystal be not too thick in proportion 

 to the intensity of the power of double refraction in the line of 

 vision. This varies much according to the position in which the 

 crystal is cut, and, therefore, in a section of a rock different crystals 

 .of the same mineral may give very different results ; but still we mav 



