MR. DOWING - Dear Sir: I am mora and more convinced of the utter uselessaess of talking about" blight."blight"What is blight? Simply the death of certain portions of a in the only way they can 4ie of any sort of sadden disease, while in full sap in warm weather, Any poison or affection that kills in hot weather, will of coarse, produce blight. However differeat the fatal cause, the sequali, from the laws of vegetation, must be the same. There may be minute differences, but the general phenomena must be similar; and whether the omasa is in the root, trunk, or branches, the tree will attempt to throw it off through- the externals and extremities, and there, to the careless observer, the disease and the death, (or blight, as it is called,) will first appear, though it may have existed for months, ox even years, in other parts of the tree, unobserved.

Again, allow me to say, that in our present state of knowledge, the world is but illy pro-pared to talk or to hear about, either the infinitely small, or infinitely great. We shall hardly be prepared to contemplate the created universe as it is, till we begin to regard with mora attention, the infinite above and below us - and to deem it possible that even man may crush microcosms of worlds beneath his puny tread, as the footsteps of Deity scatter stars and suns above and around us, I have made these remarks with reference to that form of Pear, Quince, and Apple blight, which so fearfully appeared in this state last season for the first time, and to which I alluded in my last paper, as probably caused by a microscopic insect. I am now both sad and rejoiced, to inform you that this is no longer a probability, but a certainty. Rejoiced at the fact of knowing it - but sad at the appalling nature of the feet itself, as you will see as I progress in my statement - which I hasten to communicate to the public - knowing how important it is that all eyes should at once be turned to the true cause, as the only hope of any relief - at least so far as our pears are concerned.

I shall present, at first, no theory, hut simply state facts which any one can verify at his leisure.

1. Examine the trunks and branches of apples, pears, and quinces. There will be seen little holes in the bark, sometimes two or three together, which look like the pick of an awi. These will more usually be found near the origin of a branch or sprout - and much move frequently to be seen on the apple than the pear, for reasons which will be stated.

Take a knife and cut into these perforations, and you will find the bark dead to the wood, and giving evident signs of poison, or other influence, differing from a similar wound with an awl or simple puncture, as you may learn by making the latter side by side with the former.

2. Sometime in the month of August, take out one of these pieces of punctured bark, in which the puncture has been recently made, (as the old ones were used and abondoned the year before,) and lay it in a bright sun-light, under a powerful compound solar microscope, and you will not cut many before you will discover an infinitessimal insect, somewhat resembling in legs, shape, and color, a common "sow-bag," as they are here called, running about between the fibres of the bark, much as a pig runs from side to side through the fence, to avoid the scorching heat of the sun. This discovery I made last August, as , stated in a former paper, but was still not quite certain whether this animalcule was a cause or consequence of the blight. But the discoveries of this month, I think, fully de-. cide that question.

I have been watching all winter for the results of this insect, or for its modes of action ' and transformation; But with the most careful search could discover nothing, and had m fact, almost given up in despair, when one day, by accident, I found it lay horribly apparent, right before my eyes - and that I had seen it a thousand times before without knowing it. I will, therefore, prescribe a third experiment in the process.

3. Approach that young pear tree, so healthy, thrifty, and clean - not a blemish can be found on it. Its growth, though excessive (it may be,) appears perfectly sound and good - and even the last cold winter, of 20 degrees below zero, has evidently had no power to injure in the least its glossy trunk, its bark, or even its terminal buds. All is in the brightness of a joyous and storm-defying youth. You, of course, observe little while specks on all parts of the bright brown wood - for such is its nature. But observe again; some of those little whitish specks are larger than the rest, and have the appearance of a mite of mould on the surface of the bark. Well, what strange thing is a mite of mould on a large tree? But there are more, and more, and more, and the longer you look, the more you find. Take now the point of your knife, and press one of these little insignificant patches of mould, and listen closely - you hear a crash under the knife. Ha! yon have crushed a little world. You can, even with the naked eye, see its brown blood flow beneath the relentless steel, and though it be a world of pear tree fiends, hostile to them and to you, you will find little cause to exult, when you see how many more such worlds remain unconquered around you.

Now take one of these little specks and peel it up from the bark, and apply your microscope, and you will find your speck of mould a most delicate and finely wrought texture of silk, spread as an impervious awning over a great multitude of little reddish brown eggs, of oblong form, and of beautifully smooth regular shapo and texture, still so small that scores of them are safely packed under this speck, this particle of a silken coccoon; and even on white paper - to the naked eye are scarcely visible as dust - though most beautifully apparent through the microscope.

4. But once more return to your tree; examine with a sharp knife and a microscope, and you will find on the pear, wherever the eggs are sound, and nearly ready to hatch, the bark, sometimes for one-fourth of an inch round the nidus, is already, (April 1,1863,) turned a reddish brown, in some cases quite down to the wood, and with every accession of spring warmth, it is constantly extending. These eggs, themselves, seem to be a deadly virus to the bark and sap, especially of the pear tree; and in many cases there is a hole eaten down from the eggs directly into the bark, quite to the wood of the tree, bearing all around it, the evidences of poisonous influence. Whether the parent insect emerged from these holes in the bark, or retreated into the tree through them after depositing its eggs, or simply scarified it for the benefit of its future progeny, cannot be told without future research. Where the branch is not killed, as usually happens on the apple, these holes remain, after all other appearances are gone, still visible to the naked eye.