Imatges de pÓgina


10. The

rise up in a round cylindrical form like the trunks of trees, and divide and subdivide, so as to represent branches and boughs; and has a truly wonderful and beautiful appearance. And every different kind of salt forms a different figure.

There are several other objects, which I must recommend to the attention of the young observer : as, 1. The common Fly, the vibration of whofe wings is repeated feveral hundred times in a second of tinie. The great quantity of eyes with which this animal is favoured renders it worthy of notice, each having a distinct optic nerve. 2. The Louse. 3. Mites in cheese. 4. The cuticular Pores in the human skin, so close and numerous, that a single grain of sand will cover hundreds of them. 5. The construction of the Scales of Fishes. 6. The Animalcules in several sorts of infusions and liquids. 7. The construction of common Feathers. 8. Hair. 9. The Sting of a Bee, with the form of its barbs. Configuration of Wood. 11. The common Mildew, which displays a numerous group of vegetable substances. 12. Small vegetable Seeds. And several other articles.

I fhall close this chapter with a few words concerning the method of making finall spherule lenses for microscopes, as recommended by the late celebrated Mr. George Adams. And also the method of inixing the metals for the great speculum in the reflecting telescope.

To make the finall spherule lenses, a piece of is to be cut into slips, about an cighth of an inch in breadth. Then, holding one of these flips of glass at each end in the flame of a lamp, as the glass begins to melt, it is to be drawn

by each hand into a fine thread, and at length it will break. Then one end of this thread being held in the flame of the lamp, it will run up into a small globule, which is to be taken off; and is a small spherule lens. Several of these lenses are to be made, and examined; and those that are the best, are to be preserved for use. For some will always prove faulty, though every precaution be taken.



In this process, the lamp is to be supplied with spirits of wine instead of oil, and the flame is to be blown in an hori. zontal direction by a blow-pipe, or a pair of bellows for that purpose; and the glass held in the whitest part of the flame, left the smoke fully the glass.

The metal for the speculum of the reflecting telescope is generally formed of copper and grain tin; and in the proportion of two pounds of Swedish copper, to fourteen ounces and an half of grain tin; and this mixture is to be melted twice over, before it be cast into the mould.

When the metal is cast into a concave mirror, it is ground upon, what is called, the rough griuder, or even a common grind-stone, of the same radius as the concavity of the metal, to take off the rough face. Then it is ground on a brass convex grinder, to give a true spherical figure; and lastly, upon a convex bed of hones, which is to perfect that figure, and give the metal a fine smooth face. Then the concave face of the metal is to be polished by a convex tool, covered with pitch. And, lastly, it is to be brought into the parabolic form by a merely mechanical method of grinding it on the polisher in a different direction.

But the best metal for speculums is that proposed by the Rev. Mr. Edwards, and which was proved by Dr. Maskelyne to excel, in brightness and distinctness of the image, every other metal then known. It consists of thirty-two ounces of copper, with fifteen or sixteen ounces of grain tin (according to the purity of the copper), with one ounce of brass, one ounce of silver, and one ounce of arsenic. I once was present at the casting of a speculum of this metal; and, when finithed, it reflected more light than any speculum I have erer seen.


5. Gosseli, Printer, Little Qucen Sucet, Hoiburn,

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