Note 151 — CHAP. X. OF THE CRYSTALLIZATION OF SALTS, AS SEEN BY THE MICROSCOPE; TOGETHER WITH A CONCISE LIST OF OBJECTS.

The manner of viewing the particles of the blood has been described in p. 149, together with some remarks on their form, by our author. It was not my intention to have renewed the subject; but a chirurgical treatise having been lately published by Everard Home, Esq. F. R. S. in which it appears that he has paid particular attention towards investigating these minute particles, and ascertaining their true form, I shall here subjoin an abstract.

“As the result of microscopical experiments has been found exceedingly fallacious, a prejudice has very naturally arisen against all experiments of this kind upon the secretions of the human body, from a supposition that they are not to be depended upon. But it is right that we should discriminate, and not condemn the use of the microscope altogether, because from ignorance of its principles it has been misapplied; since these very deceptions have been the means of our acquiring a more accurate knowledge of the use and application of that instrument.

“The errors in the use of the microscope have arisen from increasing the magnifying powers of the glasses too much, and not taking in all the circumstances relating to the refraction of the rays of light, making no allowance for the aberration. An attention to the aberration alone will explain the different appearances under which the red globules of the blood have been represented. Some have found them perfect spheres, which will always be the case when the glasses are perfectly adjusted, and the object placed at the true focal distance. Others have found them annular, from the object being at the focal distance of the rays transmitted near the circumference of the magnifying glass, which are refracted in a greater degree, and consequently shorter than the central rays. Others, again, have viewed them as flattened bodies of a circular figure, bright in the center, and becoming darker towards the edges; which appearance arises from the object being at the focal distance of the central rays of the magnifying glass, which will be less refracted than those near the circumference. Although such are the errors which arise, when microscopical researches are pushed beyond certain bounds; yet, that the red part of the blood is made up of globules, is a discovery for which we are indebted to the microscope, and which seems to be as well ascertained as any discovery in anatomy or physiology. The appearances of pus are equally distinct, when examined on the field of a microscope, as the globules of the blood; they are visible with a small degree of magnifying power, and are the same to the eyes of different persons.” Edit.