In these instruments two distinct functions may be observed: their light-gathering power depends on the diameter of their object mirror or lens, their defining power depends on the exquisite correctness of their optical surfaces.

Grand instruments may possess the former quality in perfection by reason of their size, but the latter very imperfectly, either through want of original configuration, or distortion arising from flexure through their own weight.

But, unless an instrument be perfect in this respect, as well as adequate in the other, it may fail to decompose a nebula into discrete points.
Fortunately, however, other means for the settlement of this question are available.

In 1846, it was discovered by the author of this book that the spectrum of an ignited solid is continuous--that is, has neither dark nor bright lines.

Fraunhofer had previously made known that the spectrum of ignited gases is discontinuous.