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1831.] then again heated to ensure the dissipation of all volatile matter.

After this is done, the platinum should be rubbed to powder by the clean finger on clean paper (83), heated slightly a third time, and then preserved in a clean and well-stopped bottle.

On a Peculiar Class of Optical Deceptions.

pre-eminent importance of the eye as an organ of perception confers an interest upon the various modes in which it performs its office, the circumstances which modify its indications, and the deceptions to which it is liable, far beyond what they otherwise would possess. The following account of a peculiar ocular deception, which, in a greater or smaller degree, is not uncommon, and which, if looked for, may be observed with the utmost facility, may therefore prove worthy of attention; and I am the more inclined to hope so, because in some points it associates with an account and explanation of an ocular deception given by Dr. Roget in the 'Philosophical Transactions' for 1825, page 121.

The following are some cases of the appearance in question. Being at the magnificent lead mills of Messrs. Maltby, two cog-wheels were shown me moving with such velocity, that if the eye were retained immovable, no distinct appearance of the cogs in either could be observed; but, upon standing in such a position that one wheel appeared behind the other, there was immediately the distinct though shadowy resemblance of cogs moving slowly in one direction.

Mr. Brunel, jun. described to me two small similar wheels at the Thames Tunnel: an endless rope, which passed over and was carried by one of them, immediately returned and passed in the opposite direction over the other, and consequently moved the two wheels in opposite directions with great