Ces irregularites memes sont des faits qu'il importe de connoitre.

LES SOINS LES PLUS ATTENTIFS N'EN SAUROIENT PRESERVER LES OBSERVATEURS LES PLUS EXERCES, et celui qui ne produiroit que des angles toujours parfaitment d'accord auroit ete singulierement bien servi par les circonstances ou ne seroit pas bien sincere."-- BASE DU SYSTEME METRIQUE, Discours Preliminaire, p.

158.
This desire for extreme accuracy has called away the attention of experimenters from points of far greater importance, and it seems to have been too much overlooked in the present day, that genius marks its tract, not by the observation of quantities inappreciable to any but the acutest senses, but by placing Nature in such circumstances, that she is forced to record her minutest variations on so magnified a scale, that an observer, possessing ordinary faculties, shall find them legibly written.

He who can see portions of matter beyond the ken of the rest of his species, confers an obligation on them, by recording what he sees; but their knowledge depends both on his testimony and on his judgment.
He who contrives a method of rendering such atoms visible to ordinary observers, communicates to mankind an instrument of discovery, and stamps his own observations with a character, alike independent of testimony or of judgment.
SECTION 2.

ON THE ART OF OBSERVING.
The remarks in this section are not proposed for the assistance of those who are already observers, but are intended to show to persons not familiar with the subject, that in observations demanding no unrivalled accuracy, the principles of common sense may be safely trusted, and that any gentleman of liberal education may, by perseverance and attention, ascertain the limits within which he may trust both his instrument and himself.
If the instrument is a divided one, the first thing is to learn to read the verniers.