POLARISED LIGHT. 1 03 



of the shifting of the paper, as, when the wood is moved, both micro- 

 scope and paper will move with it. In all sketches made by the ca- 

 mera, certain things must be borne in mind : the eye, when once 

 applied to it, should be kept steadily fixed in one position ; and if the 

 sketches are to be reserved for comparison with others, the distance 

 between the paper and the camera should be always the same. A 

 short rule or a piece of wood may be placed between the paper and 

 the under-surface either of the compound body or the arm supporting 

 it, in order to regulate the distance, as the size of the drawing made by 

 the camera will depend upon the distance between it and the paper. 

 It is also very desirable, 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 mea- 

 surement of all parts of the object. Some persons cover the whole of 

 the drawing over with squares, to facilitate, 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 accu- 

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

 the picture will depend entirely upon the skill of the artist himself. 



ON THE. POLARISATION OF LIGHT AS APPLIED TO THE MICROSCOPE. 



Huyghens and others having observed that a ray of light has not 

 the same properties in every part of its circumference, 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 posi- 

 tions j and thence the term polarisation, a term having neither re- 

 ference to cause nor effect. It is to Malus, however, who, in 1808, dis- 

 covered 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 any thing which 

 can be presented to our eyes under the microscope. It has been truly 

 observed by Sir David Brewster, that "the application of the prin- 

 ciples of double refraction to the examination of structures is of the 

 highest value. The chemist may perform 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 exquisite skill; but 

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



