2/6 SUPPLEMENT TO THE 



be evident, by a bare inspection of figure 4, that, in the 

 case of over correction for sphericity, the conditions before 

 stated relative to an uncorrected lens or object-glass must 

 be more or less reversed ; that, as the main body of the 

 light will be thrown beyond the focus (F), where there will 

 be no diffusion of light, as at the points (f f f), but a cer- 

 tain diffusion within the point F, or principal focus ; as it 

 is known to be very easy to ascertain both the figure and 

 focus of a concave of flint glass requisite to over correct 

 a given convex of crown or plate, this method has been 

 resorted to in order to procure data as to the phenomena 

 which it will produce when in combination with a convex 

 too weak for it, which are as follows : First, there is a dif- 

 fused nebulosity at the focus, (see plate 13, figure 7 /f/ , 

 dial-plate) ; (figure 7> image of a window in a globule of 

 mercury) ; figure 7', ditto, with the image of the sun), but 

 not so extensive as in the case of the uncorrected lens ; 

 figures 8' and 8 X// now represent the appearances beyond 

 the focus, and figure 10 those within it, consisting of a 

 strong burr, with annuli in the centre, which are but barely 

 visible in a small globule on a bright day, and not to be 

 recognized at all in a large one in dull weather; all fully 

 confirming the testimony of figure 4. The indications 

 caused by over correction, for colour,' will be rendered 

 evident, by a consideration of the fact, that, if a prism of 

 flint glass is made so powerful as completely to get the bet- 

 ter of another of crown or plate applied to it, so that their 

 refracting angles shall be opposed to each other, the dis- 

 persion of the flint one will alone be perceptible, and the 

 colours will in consequence be refracted in the contrary 

 order to that in which they would have appeared had the 

 prism of plate or crown an equal mastery over the flint one. 



