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In the whole population of the United States, there is a considerable excess in the proportion of the blind among the blacks over that among the whites; it being among the blacks 1 to 1584; among the whites 1 to 2650; the proportion of blind persons, blacks and whites, in all the Union being, according to this table, as 1 to 2363.

In Tennessee, however, we find more blind in a given number of whites than in the same number of blacks: the former being 1 in 3044, -- the latter 1 in 3950. In South Carolina, the proportion is about the same among blacks and whites.

How are these differences to be accounted for? Without examining in detail the theory which the compiler of the table has raised upon these calculations, -- the whole fabric may be brought to the ground by knocking away the foundations, and showing that the table is manifestly incorrect, which we believe has been done in the text.

That the proportion of blind among the blacks should be greater than among the whites, is perfectly natural and in accordance with the general principle which we have laid down, that the poor are more exposed to the causes of blindness than the rich; the blacks being generally poor.

In a statement of the number of blind in the different cities, which follows, the author of this table gives the number of blind in Boston as nearly three times greater than it really is. It is important to ascertain the proportion between the blind who are of an age to receive an education, and those whom age renders unfit for it; we believe it to be much less than is generally supposed: the number of children born absolutely blind is very small; but many become so in a few weeks or months; fewer between infancy and youth, but still more rarely is the sight lost in youth or during manhood. Old age indeed dims the vision, but it is seldom thus entirely lost. The table we have quoted gives the following proportions.

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It is impossible, however, to form any estimate of the proportional number of the blind in sections of the country so small, as those in regard to which the writer of this paper attempts to do it; nor do we agree with him in the causes which he assigns for the apparent variations. The fact is, that we cannot make any accurate calculation of the number of the blind which will be found even in a population of one million; for it varies from temporary causes, and in different generations; but we may calculate with some degree of certainty, how many blind persons will be found in a population of ten millions, the latitude and the climate being given.

How little dependence can be placed upon the calculations made in the paper to which we have referred, and in which the proportion in every county in Pennsylvania is attempted to be laid down, may be inferred from the fact that, in this city, there is but one blind person of the proper age for receiving an education; while, in the neighbouring town of Andover, with less than one twentieth of the population, there are five; in Cambridge four, and in some small towns on Cape Cod three. In the next generation, however, the prevalence of ophthalmia may give to Boston twenty or thirty: but though the laws of nature in this respect seem thus variable, they are in reality wonderfully uniform, and in every age the proportion of the blind to the whole population is about the same. Blindness appears to be more prevalent in the country than in cities, probably from the fact that people there can seldom procure medical assistance so seasonable or so efficient as to be of much use in the ophthalmia of infants. The poor are certainly more subject to it than the wealthy, partly from more exposure and partly perhaps from the hereditary nature of blindness. It is well known that blindness is very often hereditary, and we have instances in our neighborhood, of five children being born blind from the same mother; now where such a scourge enters a family, it may readily be conceived how soon it will be reduced to poverty.

We have said that the general law of nature, by which a certain proportion of the human race are born with but four senses in perfection, is unfailing in its operation; and in the want of any accurate statistics, we may correct our own by those of other countries similarly situated. As a general rule, blindness is more prevalent within the torrid zone, less in the temperate, and less still in the frigid: in dry and sandy soils it is more prevalent than in moist ones. Egypt is the country of the blind par excellence; different writers have estimated the proportion of the blind there very differently; some say, that one man in every hundred is totally or partially blind; others one in three hundred. The latter calculation is probably the nearest to the truth; but from our observation of the number of men with but one eye, or with distorted eyes in the Egyptian army, we are inclined to think that the number of the blind in Egypt must be fearfully great. The cause is probably the fine sandy dust with which the air is continually filled in Egypt; and which exists to such a degree, that the first cotton machinery sent out from England for the Pacha Mehemet Ali, was rendered useless by it in a very short time. This difficulty is the greatest which his engineers have had to overcome.