195 



A paper was read, entitled, " On the construction and use of 

 Single Achromatic Eye-Pieces, and their superiority to the double 

 eye-piece of Huyghens." By the Rev. J. B. Reade, M.A., F.R.S. 



The author observes, that experience has shown it to be impracti- 

 cable to make a telescope even approach to achromatism, by employ- 

 ing the same object-glass with an astronomical, as with a terrestrial 

 eye-piece : for if the focus of the blue rays from the object-glass be 

 thrown forwards, as it must be in order to make it impinge upon 

 the focus of the blue rays of the terrestrial eye-glass, then there will 

 be produced a great over-correction for the astronomical eye-glass ; 

 and vice versa. Hence it appears that the application of Huyghen- 

 ian eye-pieces to refracting telescopes, is incompatible with the 

 conditions of achromatism, throughout the entire range of magni- 

 fying power ; and that in reflecting telescopes they unavoidably in- 

 troduce dispersion, because they are not in themselves achromatic. 

 These defects the author proposes wholly to obviate, by substituting 

 for the Huyghenian eye-pieces, single achromatic lenses of corre- 

 sponding magnifying power; consisting of the well-known combina- 

 tion of the crown-lens, and its correcting flint-lens, having their ad- 

 jacent surfaces cemented together ; thus avoiding internal reflections, 

 and enabling them to act as a single lens. The achromatic eye-pieces 

 which he uses were made by Messrs. Tulley and Ross, and are of 

 the description usually termed single cemented triples. 



A paper was also read, entitled, " Meteorological Observations 

 made between October, 1837, and April, 1839, at Alton in Fin- 

 marken." By Mr. S. H. Thomas, chief mining agent at the Alton 

 Copper Works. Presented to the Royal Society by John R. Crowe, 

 Esq., Her Britannic Majesty's Consul at Finmarken. Communicated 

 by Major Edward Sabine, R.A., V.P.R.S. 



This memoir consists of tables of daily observations of the baro- 

 meter and thermometer, taken at 9 a.m., 2 p.m., and 9 p.m., with re- 

 marks on the state of the weather, at Kaafjord, in latitude 69° 58' 3'' 

 north, and longitude 23° 43' 10" east of Paris. 



January 16, 1840* 



JOHN WILLIAM LUBBOCK, Esq., V.P. and Treasurer, in the 



Chair. 



A paper was read, entitled, " On Nobili's Plate of Colours ; in a 

 Letter from J. P. Gassiot, Esq., addressed to J. W. Lubbock, Esq., 

 V.P. and Treasurer R.S." Communicated by J. W. Lubbock, 

 Esq. 



The efl'ect produced by the late Signer Nobili, of inducing 

 colours on a steel plate, excited the curiosity of the author, and led 

 him to the invention of the following method of producing similar 

 effects. Two of Professor Daniell's large constant cells were exci- 



