284 Dr. Turner on a Specimeri of Varvicite from Ihlefeld. 



but a space of sensible size, and the appearance is the same 

 as when, with two eyes, we hide the candle bj'a card from one 

 eye only, we may fancy that we see the candle through the card. 

 And from this fringe or indecision of outline arises the slight 

 pain which the eye sometimes feels, on looking at one piece of 

 gauze of fine network through another. 



This fringe appears and affects the eye like an object not 

 exactly in the focus of a magnifying glass. 



Exper. 6. — If the eye be directed in a piercing manner to 

 an object at six feet distance, with a view to distinguish the 

 minute parts, the lens of the eye is flattened, that is, accom- 

 modated to the purpose by having the focus made more di- 

 stant; if it be then quickly turned to a point of light nine 

 inches distant, there appears a dark spot in the middle, as in 

 fig. 10. 



Exper. 7. — If a light be seen through a coach-glass made 

 dull by breath condensed on it, the light appears surrounded 

 by a ring of the prismatic colours, the red being at the out- 

 side and the violet at the inner ring. Fig. 11. 



Exper. 8. — If the bright spot of light on a thermometer- 

 bulb be observed, the outer edge of the spot which enters the 

 eye through the iris, (see fig. 5, 6, 7, 12,) will be seen tinged 

 with red, showing that the eye is not quite achromatic, but is 

 what opticians term under-covr&cieA ; but the pupil appears 

 to be perfectly achi'omatic; though it may perhaps be unphi- 

 losophical to apply the term to the pupil ; as it is itself our only 

 standard we have no other by which we can compare it. 



Canonbury, March 2, 1830. SamUEL SharPE. 



XLI. On a Specimen of Varvicite from Ihlefeld. By Dr. 

 Edward Turner, F.R.S. L. ^ E. Sfc. Prof, of Chemistry in 

 the University of London. {In a Letter to Richard Phillips, 

 F.R.S. L. 4- E.) 

 My dear Sir, 



1HAVE lately received a curious specimen of manganese 

 from Prof. Stromeyer, in the history of which you will feel 

 considerable interest, as my examination of it leaves little or no 

 dou*bt of its being a specimen of varvicite. It was found about 

 a year ago at Ihlefeld in the Hartz, and occurs in after-crystals 

 having the form of the six-sided pyramid of calcareous spar. 

 From this circumstance it is probable that the crystals at one 

 time consisted of carbonate of manganese, which has been con- 

 verted into its present state by subsequent decomposition. 



Of the nature of the original crystals two views seem plausible. 

 They may originally have been composed of carbonate of 



lime, 



