“Two magnitudes whether commensurable or incommensurable, balance at distances reciprocally proportional to the magnitudes.”

Source: Sir William Herschel: His Life and Works (1880), Ch.4 "Life and Works" from a memoir, published (1817).
Context: It is evident that we cannot mean to affirm that the stars of the fifth, sixth, and seventh magnitudes are really smaller than those of the first, second, or third, and that we must ascribe the cause of the difference in the apparent magnitudes of the stars to a difference in their relative distances from us. On account of the great number of stars in each class, we must also allow that the stars of each succeeding magnitude, beginning with the first, are, one with another, further from us than those of the magnitude immediately preceding.

On the Hypotheses which lie at the Bases of Geometry (1873)

1860s, Allow the humblest man an equal chance (1860)
Context: Look at the magnitude of this subject! One sixth of our population, in round numbers — not quite one sixth, and yet more than a seventh, — about one sixth of the whole population of the United States are slaves! The owners of these slaves consider them property. The effect upon the minds of the owners is that of property, and nothing else — it induces them to insist upon all that will favorably affect its value as property, to demand laws and institutions and a public policy that shall increase and secure its value, and make it durable, lasting and universal. The effect on the minds of the owners is to persuade them that there is no wrong in it. The slaveholder does not like to be considered a mean fellow, for holding that species of property, and hence he has to struggle within himself and sets about arguing himself into the belief that Slavery is right. The property influences his mind. [... ] Certain it is, that this two thousand million of dollars, invested in this species of property, all so concentrated that the mind can grasp it at once — this immense pecuniary interest, has its influence upon their minds.

Act II, scene ii
Love for Love (1695)
Variant: Ferdinand Mendez Pinto was but a type of thee, thou liar of the first magnitude.

Letter http://founders.archives.gov/documents/Adams/99-02-02-7261 to William Tudor, Jr., 20 November 1819. Partially quoted in Founding Brothers : The Revolutionary Generation (2000) by Joseph J. Ellis, p. 240
1810s
Context: I Shall not pause to consider whether my Opinion will be popular or unpopular with the Slave Holders, or Slave Traders, in the Northern the Middle, the Southern, or the Western, States—I respect all those who are necessarily subjected to this Evil.—But Negro Slavery is an evil of Colossal Magnitude. … I am therefore utterly averse to the admission of Slavery into the Missouri Territory, and heartily wish that every Constitutional measure may be adopted for the preservation of it.

Letter to the The Times attacking the "People's Budget" (22 June 1909), p. 8.

Source: The Human Form: Sculpture, Prints, and Drawings, 1977, p. 10.

De Luce seu de Inchoatione Formarum (c. 1215-1220)
Context: One cause, in so far as it is one, is productive of only one effect. I do not rule out several efficient causes of which one is nearer and another more remote in the same order. Thus when I say simply 'animal', I do not exclude another substance or particular substance. Hence motion, in so far as it is one, is productive of only one effect. But motion is present in every body from an intrinsic principle which is called natural. Therefore an efficient cause simply proportional to the motion is present in all bodies. But nothing is present in common in every body except primitive matter and primitive form and magnitude, which necessarily follows from these two, and whatever is entailed by magnitude as such, as position and shape. But simply through magnitude a body does not receive motion, as is clear enough when Aristotle shows that everything that moves is divisible, not, therefore, simply because of magnitude or something entailed by magnitude is a body productive of motion. Nor is primitive matter productive of motion, because it is itself passive. It is therefore necessary that motion follow simply from the primitive form as from an efficient cause.

Source: The Gathering Storm