Mechanical Science. 133 



propriety ; for though the chromatic aberration in them was in a 

 great measure subdued, the spherical aberration was unaltered ; 

 their distinctness, tlierefore, was no greater than that of a com- 

 mon lens of the same angle of aperture. An immense difficulty 

 remained to be overcome, viz.^ the distinction of the aberration of 

 figure, together with that arising from dispersion, wliich has now 

 been effected. It is but a small point gained to render these lenses 

 free from colour, for they may, notwithstanding, be no better, or 

 even a great deal worse^ with regard to distinctness, than common 

 ones, as is the case also with the chromatic object glasses of 

 telescopes. 



Mr. J. Cuthbert has also (under the direction of Dr. Goring) con- 

 structed a reflecting microscope, on the principle of that invented^ 

 by professor Amici, of Modena, which, in its original condition and 

 dimensions^ viz.^ with an objective metal of 2\ inches sidereal Jbeus, 

 is good for nothing, notwithstanding the pompous eulogiums which 

 have been bestowed on it ; being unable to shew any but the most 

 common and easy objects, as can be demonstrated by a very excel- 

 lent one of the kind previously made by Mr. Cuthbert, in which the 

 objective metal has a truly elliptical figure, ^-c* Dr. Goring was of 

 opinion that tlie principle of the instrument was good, but that the 

 failure in the performance arose from the object metal of 2-^ inches 

 focus, with a tube 12 inches long, forming an image which was 

 only about three times larger than the object, so that all the rest 

 of the requisite power had to be obtained by very deep eye-glasses : 

 he accordingly planned both the optical and mechanical arrange- 

 ments for another instrument, which has an objective metal of 

 only .6 inches sidereal focus, and .3 aperture, with a tube 5 

 inches long. This being executed by Mr. Cuthbert, was found to 

 perform extremely well, and to exhibit any objects which could be 

 seen with the single microscope, to which it seemed equal, power 

 for power. 



Dr. Goring has also caused a diamond lens of ^^th inch focus, to 

 be executed by Mr. Andrew Pritchard, of 51, Upper Thornhaugh 

 Street, (assistant to Mr. Cornelius Varley, under whose auspices 

 it was worked.) A diamond is well known to be the most refrac- 

 tory body in nature, at least it is the most difficult which could be 

 selected to receive a spherical figure ; yet it is possible to form a 

 lens of it, as the event has shewn, and (if we neglect the obstacles 

 which present themselves in working it) seems precisely the sub- 

 stance which is most adapted to form a small microscopic magnifier, 

 for its refractive power is nearly double that of glass, while its dis- 

 persive power is no greater than that of water ; its extreme hardness 

 also ultimately causes it to receive the most exquisite figure andpo- 



* Mr. Dolland^ it is said, lias also executed one with the same results. 



