sect, iv.] DISSERTATION SECOND. 1 55 



justment is made, the object seen through the aperture 

 necessarily appears indistinct to the eye, which is then 

 adapted to a near object. 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 directed 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 accuracy 7 , caeteris pari- 

 bus, is proportional to the magnifying power. 



The application of the telescope, however, to astronomi- 

 cal instruments, was not introduced without opposition. He- 

 velius 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 de- 

 termined in its position by two fixed points at a considera- 

 ble 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 



