Gardeners Dictionary: Light

Gardeners Dictionary:
containing The Best and Newest Methods of Cultivating and Improving The Kitchen, Fruit, Flower Garden, and Nursery; As also for Performing the Practical Parts of Agriculture: Including The Management of Vineyards, With the Methods of Making and Preserving Wine, According to the present Practice of The most skilful Vignerons in the several Wine Countries in Europe.

Together with Directions for propagating and improving, From real Practice and Experience, All sorts of Timber Trees.

The Eight Edition,
Revised and Altered according to the latest System of Botany; and Embellished with several Copper-Plates, which were not in some former Editions.

By Philip Miller, F. R. S.
Gardener tothe Worshipdul Company of Apothecaries, at their Botanic Garden in Chelsea, and Member of the Botanic Academy at Florence.

Printed for the Author;

(Lontoo 1768)

LIGHT is used in various senses: 1. Sometimes it signifies that sensation which is occasioned in the mind by the view of luminous bodies.

2. For those properties in those bodies, whereby they are fitted to excite those sensations in us.

3. A certain action of the luminous body on the medium between that and the eye, by the means of which the one is supposed to act on the other, and this is called secondary Light, or derived Light, in distinction to that of luminous bodies, which is called promary or innate Light.

As to the phænomenon of light, pholosophers have explained it several ways; Arsitotle by supposing some bodies to be transparent, as air, water, ice, &c. The Cartesians have considerably refined upon this notion of Light, and own, that Light, as it exists in the luminous body, is nothing else but a power or faculty of exciting in us a very clear and vivid sensation; and Father Malebranche explains the nature of Light by a supposed analogy between it and found, the latter of which is allowed to be produced by the shaking or vibration of the sinsensible parts of the sonorous body.

But the greatest discoveries into this wonderful phænomenon have been made by Sir Isaac Newton, that the primary light consists wholly in a certain motion of the particles of the lucid body, whereby they fo not propel any fictitious matter supposed to be lodged in the hidden pores of transparent bodies, but throw off from the luminous body certain very small particles, which are emitted every way with great force. And the secondary or derived Light, not in a conatus, but in a real motion of these particles receding every way from the luminous body in right lines; and with an incredible velocity.

For it has been demonstrated by Mr. Reaumur, from the observation on the satellites of Jupiter, that the progress of Light from the fun to our earth is not above ten minutes, and therefore, since the earth is at least 10,000 of its own diameters distant from the sun, Light must run 10,000 of those diameters in a minute, which is above 100,000 miles in a second.

And if a bullet, moving with the same celerity with which it leaves the muzzle of a cannon, requires twenty-five years to pass from the earth to the sun, as Mr. Huygens has computed; then the velocity of Light will be to that of a cannon ball, as twenty-five years is to ten minutes, which is above 10,000 to 1: so that the particles of Light move above a million of times swifter than a cannon ball, from which rapidity of motion very strange effects may be produced; but Sir Isaac Newton has shewn, past contradiction, that the Light of the sun is near seven minutes in its passage to the earth, which is the space of 50,000,000 a velocity 10,000,000 times greater than that wherewith a ball flies out of the mouth of a cannon.

Sir Isaac Newton also observesm, that bodies and Light act mutually on one another: bodies on Light, in emitting, reflexing, refracting, and inflecting it, and Light on bodies, by heating them, and putting their parts into a vibrating motion, wherein heat principally consists; for he observes, that all fixed bodies, when heated beyond a certain degree, emit Light and shine, which shining, &c. appearts to be owing to the vibrating motion of the parts, and all bodies abounding in earthy and sulphureous particles, if they be sufficiently agitated emit Light, which way foever the agitation be effected.

The same great author observes, that there are but three affections of Light wherein the rays differ, viz. refrangibility, reflexibility, and colour; and those rays which agree in refrangibility, agree also in the other two, whence they may be well defined homogeneal. Again, the colours exhibited by homogeneal Light, he calls homogeneal colours, and those produced by definitions he afvances several propositions:

1. That the sun's Light consists of rays differing by indefinite degrees of refrangibility.

2. That rays, which differ in refrangibility, when parted from one another, do proportionably differ in the colours which they exhibit.

3. That there are as many simple and homogeneal colours, as there are degrees of refrangibility, for to every degree of refrangibility belongs a different colour.

4. Whiteness, in all respects, like that of the sun's immediate Light, and of all the usual objects of our senses, cannot be compounded of simple colours, without an indefinite variety of them, for to such a composition there are required rays endued with all the indefinite degrees of refrangibility, which infer as many simple colours.

5. The rays of Light do not act one on another in passing through the same medium.

6. The rays of Light do not suffer any alteration of their qualities from refraction, nor from the adjacent quiescent medium.

7. There can be no homogeneal colours produced out of Light by refraction, which are not commixed in it before, since refraction changes not hte qualities of the rays, but only separates those that thave divers qualities by means of their different refrangibility.

8. The sun's Light is an aggregate of homogeneal colours, whence homogeneal colours may be called primitive or original.

Hence proceeds the whole theory of colours in plants and flowers.

Those parts, v. g. which are the most refrangible, constitute Violet colour, the dimmest and most languid of all colours.

And, on the contrary, those particles that are the least refrangible, constitute a ray or a red colour, which is the brightest and most vivid of all colours; the other particles being distinguished into little rays, according to their respective magnitudes and degrees of refrangibility, excite intermediate vibrations, and so occasion the sensations of the intermediate colours. See Sir Isaac Newton's Doctrine of Colours.

Perhaps these observations of Light may to some persons seem foreign to the subject matter of this book, yet, if thoroughly understood might probably be found very useful. The learned and curious enquirer into to te business of vegetation, the Rev. Dr. Hales, in his treatise on that head, does, upon the query put by Sir Isaac Newton [" Are not gross bodies and Light convertible into one another? And may not bodies receive much of their activity from the particles of Light which enter their composition? The change of bodies into Light, and of Light into bodies, is very comformable to the course of nature, which seems deslighted with transmutations,"] add this query, "An may not Light also, by freely entering the expanded surfaces of leaves and flowers, contribute much to the ennobling the principles of vegetables?"

That Light has been found to be of infinite service to the growth of vegetables, has been fully proved by many experiments: 1. By painting the walls of the inside of a green-house black, whereby there will be no reflected rays of Light, when the weather becomes so cold, as that the shutters to the windows have been obliged to be kept shut a few days, the leaves of those plants which have been placed therein have dropped off.

And plants which have been placed in dark rooms, have been found totdo the same. The earthing up plants to blanch tem, whereby they become tender, and better for use; yet if these are not used, when properly blanched, will soon decay: the like will happen if plants are covered close, so as no Light can come to them, they will soon grow pale and sicken, and afterward decay.

How much the fine fracy flavour of fruits is owing to Light is hard to say, but from a few experiments it appears, most of their rich juices are beholden to Light for their excellence; therefore we may truly aver, that Light is as necessary to promote vegetation as for animal æconomy.

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