Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/318

 ed; therefore tell me how he dis-ingageth himself in the next place from that particular of the Stars having constantly kept the same distance from the fixed Stars circumjacent to it.

He betakes himself, in like manner, to two threads, yet more unable to uphold him than the former: one of which is likewise fastened to refraction, but so much less firmly, in that he saith, that refraction operating upon the new Star, and sublimating it higher than its true situation, maketh the seeming distances untain to be distinguished from the true, when compared to the circumposed fixed Stars that environ it. Nor can I sufficiently admire how he can dissemble his knowing how that the same refraction will work alike upon the new Star, as upon the antient one its neighbour, elevating both equally, so as that such a like accident altereth not the space betwixt them. His other subterfuge is yet more unhappy, and carryeth with it much of ridiculous, it being founded upon the errour that may arise in the instrumental operation it self; whilst that the Observator not being able to constitute the centre of the eyes pupil in the centre of the Sextant (an Instrument imployed in observing the distance between two Stars) but holding it elevated above that centre, as much as the said pupil is distant from I know not what bone of the cheek, against which the end of the Instrument resteth, there is formed in the eye an angle more acute than that which is made by the sides of the Instrument; which angle of rayes differeth also from it self, at such time as a man looketh upon Stars, not much elevated above the Horizon, and the same being afterwards placed at a great height; that angle, saith he, is made different, while the Instrument goeth ascending, the head standing still: but if in mounting the Instrument, the neck should bend backwards, and the head go rising, together with the Instrument, the angle would then continue the same. So that the Authours answer supposeth that the Observators in using the Instrument have not raised the head, as they ought to have done; a thing which hath nothing of likelihood in it. But granting that so it had been, I leave you to judge what difference can be between two acute angles of two equicrural triangles, the sides of one of which triangles are each four [Italian] Braces [i.e. about three English yards] and those of the other, four braces within the quantity of the diameter of a Pea; for the differences cannot be absolutely greater between the length of the two visive rayes, whilst the line is drawn perpendicularly from the centre of the pupil, upon the plain of the Rule of the Sextant (which line is no bigger than the breath of the thumb) and the length of the same rayes, whilst elevating the Sextant, without raising the head together with it, that same line no longer falleth perpendicularly upon the said plane, but inclineth, making