star could not be seen, while a larger hole made the observation too uncertain. To meet this difficulty Tycho introduced a special pinnule at the eye-end of the alidade, consisting of a square plate with a narrow slit close to the side next the alidade, while there were three other slits between the three other sides and small movable pieces of metal parallel to them. By moving these pieces the slits could be made wide or narrow according as faint or bright stars were observed. At the object-end was a small square plate exactly of the same size as the plate at the eye-end. When the alidade was pointed to a star, and the latter through the four slits was seen to touch the three sides of the object-pinnule and shine through a slit along the side next the alidade, the observer knew that the alidade accurately and without any parallax represented the straight line between his eye and the star. For observations of the sun there was in the centre of the objective pinnule a round hole through which the light fell on a small circle on the eye-pinnule, and the sunlight was generally conducted "through a canal" to keep off extraneous light. In many cases, Tycho (as we have already seen) modified the arrangement by substituting a small cylinder (perpendicular on the alidade) for the objective pinnule. On the armillæ this cylinder was placed in the centre of the axis, while the eye-pinnules could slide along the graduated circles.
Like the transversal divisions, the improved sights were introduced at Cassel by Paul Wittich, and the value of these improvements was found to be so great, that while the observers could formerly scarcely observe within 2′, the attainable accuracy was now 12′ or 14′. It appears that Wittich had not described the pinnules accurately, as he had only given them two slits instead of four, which Rothmann (or probably Bürgi) soon found preferable.[1]
- ↑ Epist., pp. 21, 28–29; T. Brahei et doct. vir. Epist., p. 100.
