380 Royal Institution. 



and he was also informed that clay-slate had been discovered more 

 to the southward in the same range. 



Isle Buache, or Garden Island, consists of the same highly cal- 

 careous sandstone which forms so considerable a portion of this 

 part of the Australian coast. 



FRIDAY-EVENING PROCEEDINGS AT THE ROYAL INSTITUTION 

 OF GREAT BRITAIN. 



Jan. 21. Mr. Faraday on a peculiar class of Optical Deceptions. 

 These deceptions depend principally upon the general effect produced 

 upon the eye when two or more bodies are presented in such rapid 

 succession to it as to produce no distinct impression for each, but pro- 

 duce a general impression often very clear and distinct in appearance, 

 but entirely unlike the real appearance of the active bodies. Thus, if 

 two equal cog-wheels be placed one before the other, and put in rapid 

 motion in opposite directions but with equal velocities, a spectral 

 fixed cog-wheel will appear -, although if either cog-wheel be looked 

 at alone, nothing but a plain uniform tint, corresponding to the place 

 of the cogs will be seen. The various deceptions depending upon this 

 effect were traced and illustrated, and it appears that many of them 

 are of common occurrence. 



In the Library numerous Wheel-animalculee were exhibited by 

 powerful microscopes belonging to Cuthbert and Varley, for the pur- 

 pose of illustrating the appearances, which were referred to the class 

 of deceptions above spoken of. 



Jan. 28. Mr. Ainsworth entered into a geological investigation 

 of the methods of determining the ages of the rocks considered as of 

 igneous origin, from a consideration of their composition and struc- 

 ture. 



Feb. 4. Mr. Brande discussed the relation of the vegeto-alkalies 

 to the common alkalies, and to certain proximate principles of vege- 

 tables. After briefly stating what Davy had done in decomposing the 

 alkalies and alkaline earths, he proceeded to detail the exertions made 

 by himself and others to obtain anything analogous to a metallic base 

 from those alkaline bodies which were known to be compounds of 

 elements not metallic. All exertions of this kind had failed j but as 

 to the effect of the Voltaic pile upon the salts of the vegeto-alkalies, 

 it was precisely the same as upon the metallo-alkaline salts, the base 

 proceeding to the negative pole, and the acid to the positive pole. The 

 properties of the new febrifuge principle Salicme were dwelt upon, 

 and also a new vegeto-alkali discovered by Mr. Hennell, but not yet 

 described, namely, Elateria. 



Feb. 1 1. Mr. Harris of Plymouth gave an account of certain in- 

 vestigations which he had made relative to the power possessed by 

 different bodies of intercepting magnetic action, and showed the ex- 

 periments by which the existence of such power was proved, and its 

 force estimated. Thus it has been supposed that iron had an intercept- 

 ing power, but copper, and many other metals and substances, none. 

 He found, however, and showed, that when the copper, silver, zinc, or 

 other substance interposed was in sufficient quantity, these metals 



also 



