284 



NA TURE 



[August 25, 1923 



The Growth of the Telescope.' 

 By Dr. William J. S. Lockyer. 



IN the beginning of the year 1608, that is, 315 years 

 ago, or about ten generations, telescopes did not 

 exist. The main work of astronomers before the year 

 1608 was, therefore, concentrated upon observing and 

 recording the positions of the heavenly bodies from 

 day to day and from year to year. The early (1587) 

 instruments for observations of position took the form 

 of graduated quadrants mounted in a vertical plane 

 capable of rotation about the centre of a horizontal 

 divided circle. The direction of a heavenly body could 

 be indicated only by pointing at it ; so every quadrant 

 was furnished with a pointer pivoted at the centre of 

 the quadrant. The adjustments of the instruments 

 were made by using a plumb line for the determination 



'cX by bUMpcii^iijU 

 can be rotated by gearing. 



of the vertical, and a level for placing the azimuth 

 circle horizontal. 



Tycho Brahe, the famous Danish astronomer (1546- 

 1601), constructed many elaborate instruments of this 

 nature for his observatory at Uraniberg, but his most 

 important instrument was the large quadrant fixed in 

 the meridian with which he observed transits of the 

 heavenly bodies through a hole in the south wall. 

 This instrument was the forerunner of the modem 

 transit circle. 



Galileo was the first to use the " optik tube " for the 

 study of the heavenly bodies, and in consequence made 

 a series of important discoveries. Thus, he found that 

 the number of stars was enormously increased ; the 

 " wandering stars " were really planets ; the moon 

 displayed mountains ; Jupiter possessed a family of 

 satellites ; Saturn exhibited curious features which 

 were eventually identified as a ring system ; Venus 

 appeared as a crescent ; spots were visible on the solar 



• From a discourse delivered at the Royal Institution on Friday evening, 

 April 20. 



surface, etc. The lens combination employed by 

 Galileo underwent changes as time advanced. In 

 1620 Kepler suggested the use of two double convex 

 lenses, and this was actually carried out by Scheiner 

 in 1637. Astronomers had to wait nearly 100 years 

 before Chester More Hall, in 1733, put fon^ard the idea 

 of making an object-glass of two different kinds of 

 glass — crown and flint — placed close together, thus 

 establishing the so-called achromatic lens. It was not, 

 however, until another quarter of a century had passed 

 that John Dollond in 1758 rendered this discovery 

 effective, thus heralding the dawn of what may be 

 termed modern astronomical observation. 



In the year 1639 the discovery of another form of 

 telescope was made, namely, the reflecting 

 telescope ; but it was not until the year 

 1663 that the principle was described in 

 practical form by James Gregory. It was 

 left, however, to Sir Isaac Newton in 1668 

 actually to construct an instrument of this 

 nature, and the telescope he made, which 

 is quite small, is to-day in the rooms of 

 the Royal Society of London. Like the 

 refracting telescope, the reflecting telescope 

 underwent various changes in the optical 

 train ; thus we have the forms now known 

 as the Newtonian, Gregorian, Cassegrainian, 

 and Herschelian. 



As soon as the refracting telescope became 

 a practical instrument it was at once 

 brought into commission for instruments 

 employed in the measurements of the 

 positions of the heavenly bodies. In fact, 

 it at once replaced " pointers." Tycho 

 Brahe's great quadrant was soon super- 

 seded by a type of instrument similar to 

 that made in 1770 by Sisson for the Kew 

 Observatory. This was an 8-foot quadrant, 

 mounted in the meridian, with a finely 

 divided scale and vernier. The quadrant 

 form developed later into a complete graduated circle 

 read by several microscopes after the type of Gambey's 

 mural circle, made in 181 9 for the Paris Observatory'. 



The acme of perfection in accuracy is reached to-day 

 by such an instrument as the present Cape Obser\-atory 

 transit circle. In this the telescope has an objective 

 of 6 inches aperture of the finest construction, two very 

 finely graduated circles are attached, and several 

 micrometers are employed for reading each circle. 

 Many other refinements, too numerous to mention 

 here, are included to attain the highest accuracy. 



In order to follow the developments of the two kinds 

 of telescopes, namely the refractors and reflectors, it 

 is best to deal with each kind separately. Returning 

 to the epoch many years before John Dollond made 

 the achromatic lens effective, it was found that an 

 object glass, which then consisted of a single lens only, 

 formed images at the focus which were highly coloured 

 and spoilt definition. The only method of securing 

 greater magnifying power, with increase of aperture 

 or diameter of lens, was to make the lenses of great 



tower which 



NO. 2808, VOL. 112] 



