Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/355

 angle in the Starry Sphere, that insisteth upon the diameter of the Star, and by the proportion of the ropes thicknesse to the distance from the eye to the rope, by the table of Arches and Chords, I have immediately found the quantity of the angle; using all the while the wonted caution that is observed in taking angles so acute, not to forme the concourse of the visive rayes in the centre of the eye, where they are onely refracted, but beyond the eye, where really the pupils greatnesse maketh them to concur.

I apprehend this your cautelous procedure, albeit I have a kind of hæsitancy touching the same, but that which most puzzleth me is, that in this operation, if it be made in the dark of night, methinks that you measure the diameter of the irradiated Discus, and not the true and naked face of the Star.

Not so, Sir, for the rope in covering the naked body of the Star, taketh away the rayes, which belong not to it, but to our eye, of which it is deprived so soon as the true Discus thereof is hid; and in making the observation, you shall see, how unexpectedly a little cord will cover that reasonable big body of light, which seemed impossible to be hid, unlesse it were with a much broader Screene: to measure, in the next place, and exactly to find out, how many of those thicknesses of the rope interpose in the distance between the said rope and the eye, I take not onely one diameter of the rope, but laying many pieces of the same together upon a Table, so that they touch, I take with a pair of Compasses the whole space occupied by fifteen, or twenty of them, and with that measure I commensurate the distance before with another smaller cord taken from the rope to the concourse of the visive rayes. And with this sufficiently-exact operation I finde the apparent diameter of a fixed Star of the first magnitude, commonly esteemed to be 2 min. pri. and also 3 min. prim. by Tycho in his Astronomical Letters, cap. 167. to be no more than 5 seconds, which is one of the 24. or 36. parts of what they have held it: see now upon what grosse errours their Doctrines are founded.

I see and comprehend this very well, but before we passe any further, I would propound the doubt that ariseth in me in the finding the concourse [or intersection] of the visual rayes beyond the eye, when observation is made of objects comprehended between very acute angles; and my scruple proceeds from thinking, that the said concourse may be sometimes more remote, and sometimes lesse; and this not so much, by meanes of the greater or lesser magnitude of the object that is beheld, as because that in observing objects of the same bignesse, it seems to me that the concourse of the rayes, for certain other re-