132 THE MICROSCOPE. 



before the camera is removed, to make a tracing in some 

 part of the paper of two or more of the divisions of the 

 stage micrometer, in order that they may form a guide to 

 the measurement of all parts of the object. Some persons 

 cover the whole of the drawing over with squares, to facili- 

 tate, not only the measurement, but in order that a larger 

 or smaller drawing may be made from it than that given 

 by the camera. It must be recollected, that an accurate 

 outline is the only thing the camera will give : the finishing 

 of the picture must depend entirely upon the skill of the 

 artist himself. 



ON THE POLARISATION OF LIGHT AS APPLIED TO THE • 



MICROSCOPE. 



Common light moves in two planes at right angles to 

 each other, polarised light moves only in one plane. 

 Common light may be turned into polarised light either 

 by transmission or reflection ; in the first instance, one of 

 the planes of common light is got rid of by reflection, in 

 the other, by absorption. Huyghens was among the first 

 to notice that a ray of light has not the same properties 

 in every part of its circumference, and he compared it to 

 a magnet or a collection of magnets ; and supposed that 

 the minute particles of which it was said to be composed 

 had different poles, which, when acted on in certain ways, 

 arranged themselves in particular positions ; and thence 

 the term polarisation, a term having neither reference to 

 cause nor effect. It is to Mains, however, who, in 1808, 

 discovered polarisation by reflection, that we are indebted 

 for the series of splendid phenomena which have since that 

 period been developed ; phenomena of such surpassing 

 beauty as far to exceed all ordinary objects presented 

 to our eyes under the microscope. It has been truly 

 observed by Sir David Brewster, that " the application of 

 the principles of double refraction to the examination of 

 structures is of the highest value. The chemist may per- 

 form the most dexterous analysis ; the crystallographer 

 may examine crystals by the nicest determination of their 

 forms and cleavage ; the anatomist or botanist may use 

 the dissecting knife and microscope with the most exqui- 

 site skill ; but there are still structui'es in the mineral. 



