456 'Royal Institution of Great Britain. 



But this construction possesses other and very remarkable ad- 

 vantages. For, first, when the correcting lens is approximately con- 

 structed on a calculation founded on its intended aperture, and on 

 the refractive and dispersive indices of its materials, the final and 

 complete destruction of colour may be effected not by altering the 

 lenses by grinding them anew, but by sliding the combination nearer 

 to, or further from, the object-glass, as occasion may require, along 

 the tube of the telescope by a screw motion, till the condition of 

 achromaticity is satisfied in the best manner possible. And, secondlv, 

 the spherical aberration may in like manner be finally corrected by 

 slightly separating the lenses of the correcting glass, whose surfaces 

 should for this purpose be figured to curvatures previously determined 

 by calculation, to admit of this mode of correction — a condition which 

 the author finds to be always possible. 



Mr. Rogers explains his construction by reference to a diagram, 

 and states the rule for the determination of the foci of the lenses of 

 the correcting glass in a formula which may be thus interpreted. 

 " The focal length of either lens of the correcting lens is to that of 

 the object-glass, in a ratio compounded of the ratio of the square of 

 the aperture of the correcting lens to that of the object-glass, and of 

 the ratio of the difference of the dispersive indices of crown and flint 

 glass, to the dispersive index of crown :" — for example, to correct 

 the colour of a lens of crown or plate glass of nine inches aperture, 

 and fourteen feet focal length (the dimensions of the celebrated tele- 

 scope of Fraunhofer at Dorpat) by a disc of flint glass three inches in 

 diameter, the focus of either lens of the correcting lens will require 

 to be about nine inches. To correct it by a four inch disc will require 

 a focus of about sixteen inches for each. 



The author then remarks, that it is not indispensable to make the 

 correcting glass act as a plane lens. It is sufficient if it be so ad- 

 justed as to have a shorter focus for red rays than for violet. If, pre- 

 serving this condition, it be made to act as a concave lens, the ad- 

 vantage procured by Mr. Barlow's construction of reducing the 

 length of the telescope with the same focal power, is secured : and 

 he considers, moreover, that by a proper adaptation of the distances, 

 foci, &c. of the lenses, we might hope to combine with all these ad- 

 vantages, that of the destruction of the secondary spectrum, and thus 

 obtain a perfect telescope. 



There was also read a portion of a paper " On the Occupation of 

 $ Piscium observed in Blackman-street, in the month of February 

 1821. References to recorded observations of occultations in which 

 peculiarities have been apparently seen, either at the moon's limb, or 

 upon her disc ; together with an enquiry how far certain hypotheses 

 seem adequate to account for the phenomena. By James South, Esq." 



PROCEEDINGS AT THE FRIDAY-EVENING MEETINGS OF THE 

 ROYAL INSTITUTION OF GREAT BRITAIN. 



April 25. — The lecture-room subject was practical sculpture, and 

 was delivered by Mr. Sievier. It consisted of an account of the 

 various processes adopted by the sculptor from the commencement 



of 



