MICROSCOPIC ILLUSTRATIONS. 277 



As the analogy between prisms and lenses is nearly per- 

 fect as to refrangibility, we may therefore assume that the 

 same phenomena will be presented by lenses under similar 

 circumstances, and the order of the colours already described 

 will therefore be reversed within and without the focus, in 

 cases where over correction has occurred; and this fact 

 may be very easily verified, like that relative to spherical 

 aberration, by applying a concave of such a focus and spe- 

 cific gravity to a convex as we know must infallibly prove 

 an over-match for it ; therefore we have only to repeat that 

 the phenomena already stated under the head of chromatic 

 aberration will apply more or less to all states of over cor- 

 rection, by reversing the appearances stated to exist in an. 

 ordinary lens within and without the focus ; that is, the 

 warm colouring will be found beyond the focus, the cold 

 tints within it; red, orange, and yellow, without; blue, 

 indigo, and purple, within ; &c. As to achromatism, it 

 must always speak for itself, being an absence of all colour 

 (save that of the secondary spectrum, which is impercepti- 

 ble in small object-glasses), both at the focus and within 

 and without it. 



MISCELLANEOUS DEFECTS OF OBJECT-GLASSES AND 

 METALS, &c. 



These chiefly consist of inequalities produced by bad 

 working and bad adjustment, all of which may be looked 

 into by putting them out of focus. Thus fig. 11, plate 13, 

 represents the appearance presented by a metal or object- 

 glass, the figure of which has not been properly preserved 

 in the working, though perhaps the spherical aberration 

 may be but trifling. If we take an example of a metal, 



