234 Transactions of the Royal Microscopical Society. 
peculiar spherical aberration, dependent on the nature of the glass 
employed. 
One other point should be stated, and that is, aplanatism is 
usually calculated only to a first approximation, though it is 
capable of being done to a second and third still nearer approxima- 
tions: consequently the conventional expressions now loosely in 
use of aplanatism and achromatism are approximate terms: the 
greater the number of lenses of different kinds of glass, the greater 
number of the lines of the spectrum can be united in one focus. 
And for those lines or 1 rays so united there exists no absolute 
achromatism and aplanatism, but a residuum more or less delicate 
must remain. Some of these ununited rays produce corresponding 
aberrations. Hence the extraordinary precision of definition pro- 
duced by stopping out the refractory spectral lines. 
