ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. C*!? 



arrows o' and d driven through the cork G, which is rigidly connected 

 to F. As the crystal is rotated the intensity of tlie o and e rays vary, 

 and this may be indicated by pushing the corresponding arrows in or 

 out of the cork G. The board H shows the position of tlie cross-wires 

 in the Microscope, and serves to hold RR in place. The analyser J, 

 and vibration arrows Ko and Ke are attached to the cork L, which slides 

 on or off RR. When properly in position for the ' crossed nicols ' con- 

 dition, the analyser is held in place against H by- the small stop M. The 

 and e rays from the crystal on entering the analyser are resolved into 

 two components, and of these only one is transmitted. The Hght leaving 

 the analyser is therefore made up of the resolved portions of o' and e', 

 and is indicated by the arrows Ko and Ke, both lying in the same plane, 

 but one behind the other. 



The way for the more rigid explanation of the birefringence colours 

 is often made easy by some such phrase as the follomng : " The two 

 resolved portions of the polarized ray enter the crystal plate perfectly 

 ' in step,' but as one is retarded more than the other while passing 

 through the crystal it falls behind, and when brought by the analyser 

 into the same plane the two rays are found to be no longer ' in step,' 

 and consequently interfere with each other." The interference can be 

 illustrated by soldering to each arrow^ a portion of a sine curve wire (as 

 at N). By varying the distance between the arrows, the quartz wedge 

 and kindred phenomena can be well shown. 



If a large cork be used for L, and the amplitudes of Ko and Ke be 

 varied as with d and e', one can illustrate the impossibihty of obtaining 

 complete extmction with uncrossed nicols. 



Pleochroism is exemplified by removing the analyser and cUpping 

 differently coloured glass plates (e.g. brown and green) over the o and e 

 slots in the crystal plate F. The o' and e arrows may be coloured 

 accordingly, and the variation of the tint as the crystal is rotated is 

 brought out well by varying the lengths of the o' and d arrows as before, 

 to give two colour intensities in all positions. A little mechanical 

 contrivance may be added to do this automatically, but would require 

 nice workmanship. 



The model may be used to illustrate several other points, but its 

 application to these ends will occur to every teacher. 



G-emmological Microscope and Dichroiscope.* — E. K. Spiegel- 

 halter, in a paper read before the National Association of Goldsmiths, 

 discusses some of the difficulties involved in distinguishing between true 

 gems and gem copies, especially when the gem copy is a synthetic stone. 

 In this case both are gems, the one having been made in nature's labor- 

 atory and the other in man's. Synthetic corundum is now made in 

 large quantities and in almost perfect shape. In the natural gem it 

 exists in two forms — the ruby and the sapphire — the distinction between 

 them is merely that of the oxides forming their colours. The 

 only difference between the natural and the synthetic gem is in the 

 structure-formation. There are few flawless gems, and it is in the 

 difference of structure of the specks or flaws existing in botli the 



* Optical and Photographic Trades Journ., xlv. (1913) pp. 289-290 (1 fig.). 



