S. VII.

Of the Skin; its Pores, and Scales.

If you cut off a small bit of the epidermis by means of a very sharp razor, and place it on the object-plate of the microscope; you will see it covered with a multitude of small scales, so exceedingly minute, that, according to Lewenhoek, a grain of sand would cover two hundred of them: that is to say, in the diameter of a grain of sand there are 14 or 15. These scales are arranged like those on the back of fishes, or like the tiles of a house; that is, each covering the other.

If you are desirous of viewing their form with more convenience, scrape the epidermis with a penknife, and put the dust obtained by these means into a drop of water: you will then observe that these scales, in general, have five planes, and that each consists of several strata.

Below these scales are the pores of the epidermis, which when the former are removed may be distinctly perceived, like small holes pierced with an exceedingly fine needle. Lewenhoek counted 120 in the length of a line; so that a line square, 10 of which form an inch, would contain 14400; consequently a square foot would contain 1440c0000, and as the surface of the human body may be estimated at 14 square feet, it must contain 2016 millions.

Each of these pores corresponds in the skin to an excretory tube, the edge of which is lined with the epidermis. When the epidermis has been detached from the skin, these internal prolongations

of the epidermis may be observed in the same manner as we see in the reverse of a piece of paper, pierced with a blunt needle, the rough edge formed by the surface, which has been torn and turned inwards.

The pores of the skin are more particularly remarkable in the hands and the feet. If you wash your hands well with soap, and look at the palm with a common magnifier, you will see a multitude of furrows, between which the pores are situated. If the body be in a state of perspiration at the time, you will see issuing from these pores a small drop of liquor, which gives to each the appearance of a

fountain.

§. VIII.

Of the Hair of Animals.

The hairs of animals, seen through the microscope, appear to be organized bodies, like the other parts; and, by the variety of their texture and conformation, they afford much subject of agreeable observation. In general, they appear to be composed of long, slender, hollow tubes, or of several small hairs covered with a common bark; others, such as those of the Indian deer, are hollow quite through. The bristles of a cat's whiskers, when cut transversely, exhibit the appearance of a medullary part, which occupies the middle, like the pith in a twig of the elder-tree. Those of the hedge-Hog contain a real marrow, which is whitish, and formed of radii.

As yet however we are not perfectly certain in regard to the organization of the human hair.

Some observers, seeing a white line in the middle, have concluded that it is a vessel which conveys the nutritive juice to the extremity. Others contest

this observation, and maintain that it is merely an optical illusion, produced by the convexity of the hair. It appears however that some vessel must be extended lengthwise in the hair, if it be true that blood has been seen to issue at the extremity of the hair cut from persons attacked with that disease called the Plica Polonica. But quere, is this observation certain ?

§. IX.

Singularities in regard to the Eyes of most Insects.

The greater part of insects have not moveable eyes, which they can cover with eye-lids at pleasure, like other animals. These organs, in the former, are obsolutely immoveable; and as they are de prived of that useful covering assigned to others for defending them, nature has supplied this deficiency by forming them of a kind of corneous substance, proper for resisting the shocks to which they might be exposed.

But it is not in this that the great singularity of the eyes of insects consists. We discover by the microscope that these eyes are themselves divided into a prodigious multitude of others much smaller. If we take a common fly, for example, and examine its eyes by the microscope, we shall find that it has on each side of its head a large excrescence, like a flattened hemisphere. This may be perceived without a microscope; but by means of this instrument

these hemispheric excrescences will be seen divided into a great number of rhomboids, having in the middle a lenticular convexity, which performs the part of the crystalline humour. Hodierna counted more than 3000 of these rhomboids on one of the eyes of a common fly; M. Puget reckoned 8000 on each eye of another kind of fly, so that there are some of these insects which have 16000 eyes; and there are some which even have a much greater number, for Lewenhoek counted 14000 on each eye of another insect.

These eyes however are not all disposed in the same manner: the dragon-fly, for example, besides the two hemispherical excrescences on the sides, has between these two other eminences, the upper and convex surface of which is furnished with a multitude of eyes, directed towards the heavens. The same insect has three also in front, in the form of an obtuse and rounded cone. The case is the same with the fly, but its eyes are less elevated.

It is an agreeable spectacle, says Lewenhoek, to consider this multitude of eyes in insects; for if the observer is placed in a certain manner, the neighbouring objects appear painted on these spherical eminences of a diameter exceedingly small, and by means of the microscope they are seen multiplied, almost as many times as there are eyes, and in such a distinct manner as never can be attained to by

art.

A great many more observations might be made in regard to the organs of insects, and their wonderful variety and conformation, but these we shall reserve for another place.

S. X.

Of the Mites in Cheese, and other Insects of the same kind.

If you place on the object-plate of the microscope some of the dust which is formed on the rind and other neighbouring parts of old cheese, it will be seen to swarm with a multitude of small transparent animals, of an oval figure, terminating in a point, and in the form of a snout. These insects are fur nished with eight scaly, articulated legs, by means of which they move themselves heavily along, rolling from one side to the other; their head is terminated by an obtuse body in the form of a truncated cone, where the organ through which they feed is apparently situated. Their bodies, particu, larly the lateral parts, are covered with several long sharp-pointed hairs, and the anus, bordered with hair, is seen in the lower part of the belly.

There are mites of another kind which have only six legs, and which consequently are of a dif ferent species.

Others are of a vagabond nature, as the observer calls them, and are found in all places where there are matters proper for their nourishment,

This animal is extremely vivacious; for Lewen, hoek says that some of them, which he had attached to a pin before his microscope, lived in that manner eleven weeks,