PJiiCnomcna of certain Cri/stah. 289 



being, in fact, tufts or stars of acicular crystals in such close 

 assemblage as to be in optical contact with each other and to 

 produce the appearance of a single individual. Now let us 

 suppose a group of these circles to be under examination 

 with the polarizing microscope, and when the polarizers* are 

 crossed, we observe the following phcenomenon. The field of 

 view being dark, the little circles become luminous, and we 

 see upon each of them a well-defined and dark cross, dividing 

 the crystal into four equal parts. Ail these crosses are placed 

 similarly, and are parallel to each other, and their direction 

 remains unaltered when the crystals are turned round in their 

 own plane by revolving the plate of glass upon which they 

 stand. This beautiful appearance can be seen with a mode- 

 rate magnifying power. I measured the diameter of some of 

 the larger crystals, which I found to be from jijj to g^i^ of an 

 inch. But there are many much smaller, and indeed they 

 may be seen decreasing in size, until nothing remains visible 

 of their structure but the four luminous quadrants, appearing 

 like four minute dots of coloured light placed close together. 



I proceeded to examine the circles with a high magnifying 

 power, and under favourable circumstances of illumination, 

 and 1 observed in them a very admirable structure. 



Each circle has upon it one or more coloured rings arranged 

 concentrically, but the number as well as the colour of these 

 rings is different in different individuals. 



The innermost ring is deeply coloured or black, and in- 

 closes a central space of white light, which is traversed by 

 the arms of the cross intersecting in the centre. This part of 

 the cross, which stands within the innermost ring, is beautifully 

 well defined, and perfectly black. The general appearance 

 resembles the figure 98, in Brewster's Optics, which is a re- 

 presentation of the rings seen in uniaxal crystals. It espe- 

 cially resembles it in the circumstance above mentioned, viz. 

 the more defined outline of the part of the cross which is 

 within the innermost ring. 



We have hitherto supposed the polai'izers to be crossed' 

 but if we place them in a parallel position we shall see a phae- 

 nomenon complementary to the above. The circle now pre- 

 sents four patches of coloured light, one in each quadrant ; 

 and we generally see near the centre four black or obscure 

 spots, which correspond to the arms of the cross in the other 

 position. 



Such is an outline of the microscopic appearances presented 



• IJy this term, for the sake of brevity, I here designate the polarizing' 

 and analysing prisms of single-image calcareous spar, or the plates of tour- 

 maline which may be employed in their steatl. 



Third Series. Vol.9. No. 54. Oc/. 1836. 2L 



