IT. 



THE COMPONENT PARTS OE THE MICROSCOPE. 



1. THE OBJECTIVE. 



WE will now turn to the determination of the cardinal points of 

 an objective-system, consisting of three double-lenses, each of 

 which is composed of one plano-concave flint lens and of an 

 equi-convex crown lens. Let the refractive index of the flint be 

 1*6 and that of the crown 1*5. Under these suppositions we obtain 

 for a flint-glass lens, if the thickness d and the radius of curvature 

 r be left provisionally undetermined, according to the formulae 

 contained on p. 19 et seq., 



and, hence, 



f) 



, 7 = l,/, = , t '= 57 ,,/ = l 

 consequently 



E = NO + \d , E* = N* , f> = - \r'. 



O O 



With a plano-concave lens, therefore, the second principal point 

 coincides with the posterior lens-vertex, while the first is at a dis- 



j 5 



tance of - (therefore, here, = ^ d) from the plane anterior surface. 



n O 



The focal length/ is identical with that of the curved surface ; it 



1 r 



is negative and = 7 = - r (cf. p. 22). 

 u n 1 



Similarly, we obtain for a crown-glass lens, if the radius of 

 curvature is denoted by r and the thickness by d, 



hence 



