28$ SUPPLEMENT TO THE 



"The longer focus is the place at which to ascertain 

 the utmost aperture that may be given to the glass, and 

 where in the absence of spherical error its exact state of 

 correction as to colour is seen most distinctly.' 5 



P. 197. " One other property of the double object-glass 

 remains to be mentioned, which is, that when the longer 

 aplanatic focus is used, the marginal rays of a pencil not 

 coincident with the axis of the glass are distorted/ 5 (c and 

 d, 15, plate 13,) " so that a- coma is thrown outwards, 

 while the contrary effect of a coma directed towards the 

 centre of the field is produced by the rays from the shorter 

 focus. These peculiarities of the coma seems inseparable 

 attendants on the two foci, and are as conspicuous in the 

 achromatic meniscus as in the plano-convex object-glass." 



In the original paper will be found a number of curious 

 particulars, which the reader cannot be too strongly recom- 

 mended to consult. 



METHOD OF MEASURING THE ANGLE OF APERTURE OF 

 OBJECT-GLASSES, METALS, AND LENSES. 



We have already seen that all object-glasses and lenses 

 may have their outside rays cut off, and their apertures re- 

 stricted, until they shew no sensible aberration, however 

 bad their figures and quality may be ; it therefore becomes 

 of the last importance that we should know what aperture 

 they really have, since that is the measure of their value 

 and effectiveness when associated with aplanatism and 

 achromatism. The apertures of lenses and object-glasses, 

 used for microscopic purposes, must of course be deter- 

 mined by the size of the pencil of light they admit, since 

 they operate on diverging rays ; that is, we must measure 



