SELENITE. 223 



of a Dyticus. However, the effect of polarized light is not 

 only beautiful to the eye,, but of real use to the investigator 

 of tissues, and in the researches of the pathologist, for by 

 it, the true structure of organic bodies may often be made 

 clear, when the ordinary white light has failed to develop it. 



Hardly in a concise manner can the question be an- 

 swered, which is so often asked, " Why are these objects sa 

 coloured ?" and "What is polarized light ? But I may briefly 

 explain that rays of light reflected from a body under special 

 conditions, or transmitted through certain transparent crys- 

 tals, undergo such change in their properties, that they are 

 no longer subject to the same effects of reflection and re- 

 fraction as before. 



The common ray of light may be compared to a glass 

 rod smooth and white, uniform in texture, whilst the 

 polarized ray is smooth on one side, rough and dark on 

 the other. 



How it becomes so, requires too long a dissertation on 

 the laws of light and colour ; but so it is. And when this 

 polarized ray is either thrown upon or transmitted through 

 various substances, it is either reflected, or absorbed and ex- 

 tinguished according to the structure of the object presented 

 to it. The most brilliant colours are developed by this 

 process, especially in crystals, feathers, sections of quill, 

 bone, hoof, horn. A good selection of these objects is of 

 value to the microscopist. 



SELENITE, 



which is a mineral substance consisting of crystallized 

 hydrated sulphate of lime, when split into thin layers or 

 laminae are very beautiful under polarized light, and disks 

 of blue or red selenite are used to enhance the colour of 

 objects for the polariscope. There are many crystals and 

 organic substances whose thickness is not suitable for the 

 production of distinct colour, which when a plate of selenite 

 is placed beneath them, exhibit a brilliant array of the most 

 gorgeous hues. A good disk mounted in brass costs five 

 shillings, but smaller ones may be obtained at Baker's for 

 one shilling, and answer every purpose. 



