1911-1912.] Iceland Spar and the Polariscope. 445 



spar to be divided into two equal portions : such a division 

 would be a difficult task, but if two similar pieces of spar be 

 taken, it will be a very easy matter to remove one half of 

 each of them, either by grinding or by the action of a file." 

 (I may here state that this is very much taken advantage of, 

 owing to the scarcity of spar, and by this means we are 

 enabled to use smaller pieces of spar, contenting ourselves 

 by getting half a prism out of each.) " The piece of spar 

 should not be much less than one inch long, and it need not 

 be greater than 1'4 inch; if the latter dimension be adopted 

 for the breadth, the thickness will require to be about '48 

 inch. In cementing the two pieces together, it will be well 

 to let the pointed end of the one project a little over the 

 terminal plane of the other, for by so doing a firmer contact 

 is ensured at the edges. When the cement is sufficiently 

 indurated the whole of the projecting parts may be easily 

 removed according to their cleavages. The lateral planes 

 should be left quite rough and given a coat of dead black 

 varnish to prevent the reflection of extraneous light." It can 

 now be mounted in cork, and used either as an analyser or a 

 polariser. Besides the Nicol, there are other forms of prisms 

 of more recent invention, such as the Glan-Thomson, Pras- 

 mowski, &c., but it is doubtful if they are any better than the 

 Nicol. 



Now we come to the application of the prism to the micro- 

 scope. Generally speaking, one prism is mounted below the 

 stage, it being the polariser, and the other immediately above 

 the objective, it being the analyser : often the latter is 

 mounted above the eyepiece. Although by far the most im- 

 portant applications of the microscope are those by which the 

 structure and actions of organised beings are made known to 

 us, still there are many mineral substances which make very 

 interesting and beautiful objects, being remarkable either for 

 the beauty of their structure or for their lovely colours, or 

 both combined. Dr Carpenter informs us that the natural 

 forms of inorganic substances, when in any way symmetrical, 

 are so owing to that wonderful arrangement of their particles 

 which is termed crystallisation ; and each substance crystal- 

 lises after a certain type or plan, the identity or difference of 

 these types furnishing valuable characters to the mineralogist. 



