MICEOSCOPICAL DRAWING. 133 



Microscopical Drawing. The proper method of draw- 

 ing microscopic objects is acquired by looking down 

 the tube of the microscope with one eye (preferably 

 the left) , and at the paper on which the drawing is to 

 be made with the other. Place the microscope in the 

 horizontal position, having first secured the object to be 

 copied to the stage, focus it carefully, and take care not 

 to place it too centrally, but as far towards the right as 

 it will go without taking it out of the field of view. If 

 the right eye is now opened, while the other is looking 

 down the tube, the object will be seen projected on the 

 paper, and can thus be easily traced in all its details. 



The Polarisation of Light. 



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 Malus, 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 structures in the mineral, 



