Account of another Case of Spectral Illusion. 319 



Hence f of 9>B5 will be the longitudinal aberration of the dia- 

 mond lens, viz. 108 ; and I of 758 that of the glass, viz. 884 ; 

 or in other words, the diamond will only possess about one- 

 ninth of the actual aberration of a glass lens of the same power 

 and aperture. * It will therefore be obvious, that the diamond 

 gains its advantage in two ways, first, its spherical aberration, 

 enunciated in terms of its own thicJcness, is far less than that of 

 glass ; and, secondly, this said thickness is also far less than 

 that of a glass lens of the same power and aperture, and these 

 two quantities compounded express its actual aberration. 

 Again, it must not be forgotten, that the violent refraction of 

 the diamond (which is the cause of its faint spherical aberra- 

 tion,) happens to be associated with a dispersive power also 

 lower than that of glass ; for, had its dispersion been in pro- 

 portion to its refraction, so much colour would have been ge- 

 nerated by it, as to counterbalance the advantage of its low 

 spherical aberration. I regret that J have not as yet intro- 

 duced a perfect plano-convex of diamond to the notice of the 

 public, but I am now on the point of supplying that defect, 

 and trust, from the fair promise of perfection given by the 

 stone in its flat state, when it showed no traces of flaws or poi 

 larization, that it will turn out satisfactorily. 



312, Strand, Andrew Pritchard. 



4}th February 1830. 



Art. XIX. — Account of another remarkable Case of Spectral 

 Illusion. Continued from Art. IV. p. ^22 of this Number, 



It was nearly a month after the last occurrence, that Mrs 

 was preparing for bed at about eleven at night, after a some- 

 what fatiguing drive during the day, and sitting before the 

 dressing-glass occupied in arranging her hair. She describes 

 her state of mind at the time as listless and drowsy, but fully 



* It should be remarked, that, to give the greatest eflfect to the diamond, 

 it must be formed into a meniscus lens, having the radii of its surfaces as 

 2 to 5 nearly, when the aberration would be reduced greatly below that 

 of a plano-convex. — vide Mr Coddington, page iii. 



