in the Choroid Coat of the Eye. 289 



behind the retina of the dog, and other animals which pro- 

 duce those blue and green and sometimes red reflections, which 

 often shine with such brilliancy in the living animal, loses the 

 power of reflecting these tints when it is dry, and becomes 

 entirely black. Upon learning this fact from Dr. Drummond, 

 we prepared several eyes that reflected these colours with great 

 vivacity, and invariably found that they became black when 

 dry, and blue and green when softened with water. After some 

 of them had remained four or Jive years in a dry state, they 

 still possessed the property of developing their colours by 

 moisture. It is worthy of remark, that the black passed in- 

 stantly into a brilliant blue, the blue into green, and the green 

 into a greenish yellow*." 



After this paragraph was written, I had occasion to repeat 

 the experiment on one of the eyes just mentioned, and I found 

 that the colours could be revived in the membrane ten or 

 twelve years after it had been taken from the animal. 



I do not recollect to have anywhere seen it stated that the 

 brilliant colours which appear in the eyes of animals have been 

 noticed in the human eye; but I had occasion many years ago 

 to observe them in the most distinct manner, and to examine 

 them repeatedly in the eye of a boy about ten years of age. 

 The colour was a bright red, with a purplish tinge; but it is not 

 in my power to ascertain now whether or not this colour va- 

 ried with an increase of years. On this point Mr. Fielding 

 remarks, " As regards the human eye, I have had very little 

 opportunity for investigation; and though I have proved its 

 [the new membrane's] existence, / cannot say that it ever pre- 

 sented any distinctly coloured appearance." 



Having just succeeded in finding one of the prepared speci- 

 mens of the tapetum of an ox's eye, mentioned in a preceding 

 paragraph, and which has been preserved for nearly twenty 

 years, I find that when dry, it is as black as charcoal ; and that 

 the blue and green colours of the membrane above referred 

 to, could be revived in all their original brightness by immer- 

 sion in water. 



It is a curious circumstance in the colours thus produced, 

 that though they are apparently those of thin plates, they 

 advance immediately from black to blue and green of the se- 

 cond order, all the intermediate colours of the first order be- 

 ing omitted. The same phenomenon occurs in the peacock's 

 tail, in the plumage of different birds, and in Labrador felspar. 

 In another communication I hope to be able to give a satis- 

 factory explanation of this remarkable interruption of con- 

 tinuity. 



Belleville, by Kingusie, Sept. 18, 1833. 



* Edinb. Encycl. vol. xv. p. 623. 

 Third Series. Vol. 3. No. 16. Oct. 1833. 2 P 



