Mechanical Science. 221 



from which it would appear that the flattening of the planet is 0.078, 

 or TS^.TT' *^^^ laro-er diameter bein*^ taken as ninety. M. Struve 

 has ascertained, by direct measurement, that the irrej^ular appear- 

 ance of ellipticity sometimes presented by the disc of this planet, is 

 merely an optical illusion. 



Of the satelHtes, the third had evidently the largest diameter, and 

 was nearly the same in that respect as the fourth, but it is decidedly 

 inferior in luminosity to all the others, and sometimes had a very 

 pale appearance. The first satellite is rather larger than the se- 

 cond. The following are the results obtained by actual measure- 

 ment : — 



Diameter of the first satellite 1.018 



„ „ second .... 0.914 



„ „ third 1.492 



„ „ fourth 1.277 Bih, Univ. xxxiii. 97 



2. Diamond Microscope. — Of all the various substances furnished, 

 either by nature or art, the diamond seems to be that most pre- 

 eminently calculated to form small deep lenses for single microscopes, 

 possessing a most enormous refractive power, combined with a low 

 dispersive one, together with a very little longitudinal aberration. 

 Mr. Andrew Pritchard, 18, Picket-street, Strand, has succeeded in 

 forming a very thin double convex lens, of equal radii, and about 

 l-25th inch focus, from a very perfect stone of the finest water. Its 

 polish is very beautiful, and, by its strong reflective power, at once 

 informs us of the peculiar and invulnerable material of which it 

 consists, while the large angle of aperture which it bears attests 

 the faintness of its spherical and chromatic aberration. It appears 

 from experiment, that, though the refractive power of different 

 stones varies considerably, if a diamond and a piece of plate-glass 

 are ground in tools of the same figure and radius, the magnifying 

 power of the former will surpass that of the latter, at the rate of 

 eight to three ; so that if the power of tiie glass -microscope should 

 be 24, that of the diamond one will be 64 ! What a lifl does this 

 give us in the construction of deep single microscopes ! If a dia- 

 mond was ground in the same tools which had produced a glass- 

 lens of 1-7 5th of an inch focus, (which it would be very possible to 

 do, and which we believe Mr. P. would undertake to effect,) it would 

 turn out about l-200th of an inch focus ; but this is by no means 

 the only valuable property it would possess ; for Mr. G. Francis 

 has been at the pains of calculating the value of the spherical aber- 

 ration of a plano-convex diamond, with its curvature exposed to 

 parallel rays, and finds it only 0.949 of its thickness, while that of 

 glass is known to be 1.166, cceteris paribus. This difference 

 would be very considerable, even if the thickness of a glass and 

 diamond lens of the same focus and diameter was the same ; but 

 this is, of course, i^ar from being the case, owing to the inmiense 

 refraction of the diamond, which gives u very short focus, with a 



