364 Royal Society. 



perincumbent pressure of the sea, aided by the huge materials of the 

 wreck of the frigate, which, under the influence of the swell, had acted 

 like a paviour's hammer, with enormous momentum, had jammed to- 

 gether the rocks, and produced a strong cohesion between the frag- 

 ments of wood, and the gold, silver and iron. 



The first object was to clear away every portion of the wreck ; and 

 after this had been accomplished, to loosen and remove all the large 

 rocks in succession, beginning with the smallest, and ending with the 

 largest and most unwieldy. Some of these, which they succeeded in 

 rolling from their situations into deeper water, weighed about thirty 

 or forty tons ; and the largest, which required the greatest efforts to 

 move from its place, was computed to weigh sixty-three tons. This 

 last effort served to show, that no part, either of the wreck or the 

 stores, which was of any value, remained behind ; and after fifteen 

 sixteenths of the property had been recovered, the enterprise, which 

 had so perfectly succeeded, terminated on the 24th of July, and the 

 Algerine returned to Rio de Janeiro on the 1st of August. 



The author subjoins an account of the currents off Cape Frio, and 

 a description of the climate, which seems to have been favourable, for 

 his party suffered but little from sickness, and the expedition was 

 unattended with the loss of a single life. On one occasion the party 

 were visited by a whale, which approached very near the diving-bell, 

 but fortunately changed its course, without doing any mischief. 



A paper was then read, entitled, " An Account of a Concave Achro- 

 matic Lens, adapted to the Wired Micrometer, which has been named 

 Macro-micro, from its power to increase the primary image of a Te- 

 lescope without increasing the diameter of the wires in the Micro- 

 meter." By George Dollond, Esq., F.R.S. 



The application of a concave achromatic lens to the wired micro- 

 meter of a telescope, arose out of the series of trials that were made 

 for the purpose of correcting the aberrations of the eye-glasses applied 

 to the telescope constructed by the author for the Royal Society, with 

 a fluid correcting-lens, on the plan suggested by Professor Barlow. 

 The concave lens, being interposed between the object-glass and the 

 eye-glass, and being at the same time achromatic, combines the ad- 

 vantages of doubling the magnifying power, without a corresponding 

 diminution of light, and without altering the apparent distances of 

 the threads of the micrometer. The results of the trials made with 

 telescopes to which this addition was made, are given in a letter to 

 the author from the Rev. W. R. Dawes, of Orniskirk ; from which it 

 appears that Mr. Dollond's method was attended with complete suc- 

 cess. Mr. Dawes states, that, in order to put its illuminating power 

 to a severe test, he had examined with this instrument the satellites 

 of Saturn and the minute companion of k Geminorum, but could dis- 

 cover no decided difference in the apparent brightness of the former, 

 allowance being made for the difference in the power employed ; and 

 the latter star was seen quite as distinctly with a much smaller power. 



Extracts, are subjoined from a letter of Professor Barlow's to the 

 author, containing formulae for the construction of the lens. 



