SECOND DISSERTATION. - 99 • since been called a micrometer. By introducing into the focus of the telescope a round aperture of a given size, he contrived to measure the angle which that aperture subtended to the eye, by observing the time that a star placed near the equator required to traverse it. When the angle subtended by any other object in the telescope was to be measured,. he introduced into the focus a thin piece of metal, just sufficient to cover the object in the focus. The proportion of the breadth of this plate, to the diameter of the aperture for merly measured, gave the angle subtended by the image in the focus of the telescope. This contrivance is descrled in the Systema Saturnium, at the end.
The telescope has farther contributed materially to the accuracy of astronomical observation, by its application to instruments used for measuring, not merely small angles, but angles of any magnitude whatever. The telescope here comes in place of ' the plain sights with which the index or allidad of an instrument used to be directed to an object, and this substitution has been accompanied with two advantages. • The disk • ' of a star is never so well defined to the naked eye as it is in the telescope. Besides, in using plain sights, the eye adapts itself to the farther off of the two, in order that its -aper ture may be distinctly seen. Whenever this adjustment is made, the object seen through the aperture necessarily appears indistinct to the eye, which is then adapted to a near ob ject. This circumstance produces an uncertainty in all such observations, which, by the use of the telescope, is entirely removed.
But the greatest advantage arises from the magnifying power of the telescope, from which it follows, that what is a mere point to the naked eye, is an extended line which can be divided into a great number of parts when seen through the former. The best eye, • when not aided by glasses, is not able to perceive an object•which subtends an angle less than half a minute, or thirty seconds. When the index of a quadrant, therefore, is direct ed by the naked eye to any point in the heavens, we cannot be sure that it is nearer than half a minute on either side of that point. But when we direct the axis of a telescope, which magnifies thirty times, to the same object, we are sure that it is within the thirtieth part of half a minute, that is, within one second of the point aimed at. Thus the acme
racy ceteris paribus is proportional to the magnifying power.
The application of the telescope, however, to astronomical instruments, was not intro. duced without opposition. Hevelius of Dantzic, the greatest observer who had been since Tycho Brahe, who had furnished his observatory with the best and largest instruments, • and who was familiar with the use of the telescope, strenuously maintained the superiority of the plain sights. His principal argument was founded on this,—that, in plain sights, the line of collimation is determined in its position by two fixed points at a considerable distance from one another, viz. the centres of the two apertures of the sights, so that it remains invariable with respect to the index.
In the case of the telescope there was one fixed point, the intersection of the wires in the focus of the eye-glass ; but Hevelius did not think that the other point, viz. the op. tical centre of the object-glass, was equally well defined. This doubt, however, might have been removed by a direct appeal to experithent, or to angles actually measured on the ground, first by an instrument, and then by trigonometrical operations. From thence it would soon have been discovered, that the centre of a lens is in fact a point defined more accurately than can be done by any mechanical construction.
This method of deciding the question was not resorted to. Hevelius and Hooke had a very serious controversy concerning it, in which the advantage remained with the lat ter. It should have been observed that the French astronomer, Picard, was the first who employed instruments furnished with telescopic sights, about the year 1665. It appears, however, that Gascoigne, an English gentleman who fell at the battle of Marston-moor in 1644, had anticipated the French astronomer in this invention, but that it had remained entirely unknown. He had also anticipated the invention of the micrometer. The vast additional accuracy thus given to instruments formed a new era in the history of astrono mical observations.