On the Affections of Light. 323 



the leaves assume the deepest yellow ; and when very dry, a dirty 

 light red, which is undoubtedly the iron in the state of an oxide. 

 Biit 1 in vain endeavoured, by waterincr different species of plants 

 with a very dilute solution of iron in different acids, to obtain a 

 greater degree of colour in the leaf; and I further made this ex- 

 periment o"n flov.ers and .such plants whose leaves were not of a 

 direct green, and with different metals in solution ; but was not 

 able without the destruction of the plant to alter the colour, 

 nor did the plants when submitted to a rough analysis appear 

 to l)e the least altered in respect to their composition. 



This was, perhaps, the delusive chain of tlie alchemists: but if 

 followed up by some of your scientific readers in these more en- 

 lightened time's, where actual experiment is unbiassed by super- 

 htition, it might throw much light on what, from being familiar to 

 us, is in a great measure overlooked as a common phcenomenon. 

 I anij sir, respectfully, 



Your constant reader, 



Stoke Ncwiiij^ton. P. N.- J. 



»• 



LIV. On the affections of Light transmitted through cry- 

 stallized Bodies. By David Brewster, LL.D. F.R.S. 

 Edin. and F.S.J. Edin. In a Letter to Sir Humphrt 

 Davy, LL.D. F.R.S. 



[Concluded from p. 370.] 



V. On the elliptical coloured Rings produced hj depolarizing 

 Crystals. 



In a former work, to which I have already had occasion tg 

 refer, I have given some account of the colours Avhich accom- 

 pany the dei)olar;/;ition of light, and I have particularly noticed 

 the 'remarkable face, that wiieu a beam of white depolarized 

 light is transmitted tlirough a doubly refracting crystal, the red 

 rays go to the formation of one image, while the blueish-greea 

 rays go to the formation of the other image. In repeating and 

 extending these experiments, I have been led into a new field ot 

 inquiry, which has already afforded a series of instructive re- 

 sults deduced from a class of phaenoniena unquestionably the 

 most brilliant within the whole range of optics. 



The plate of topaz which v/as used in these cjqjeriments, is 

 about -j-LO^Sy of an inch thick, and has two natural faces which are 

 parallel and highly polished. Us refractive power is 1-G3G ; its 

 dispersive power U'U24, and the angle at which it polarizes light 

 by reflection 5S'-' 8', It is represeuted in section by AEab, ia 

 Plate IV., fig. 8, DE being one of its depolarizing axes. If a 

 beam of common light Rll' is now incident on the anterior sur- 

 face AB at an angle of about 60' 38', a part of the beam will 



X 2 penetrat* 



