268 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



them to be indistinct. Herscbel found that in England there were 

 not more than one hundred hours during which the sky could be 

 studied usefully with his largest telescope, using a magnifying power 

 1000 times. This conclusion drove the celebrated astronomer to 

 acknowledge that to take with his instrument a survey of the 

 heavens, with the field only one instant on each point of the 

 expanse, would require no less than eight hundred years. 



The telescope which was erected by Lord Kosse in his park of 

 Parsonstown, in Ireland, is still more colossal than Herschel's. The 

 metal mirror of 1/83 metres (6 ft.) in diameter and about 17 metres (60 

 feet) focal length, alone weighs nearly 4000 kilogrammes. The total 

 weight of the optical apparatus, tube and mirror, is not less than 

 10,400 kilogrammes. It has been stated that a magnifying power 

 of 6000 can be employed. But such a power is only applicable to 

 observations of very luminous objects, such as stars or star-clusters. 

 Eesearches in sidereal astronomy have been carried on with the 

 greatest success by means of this magnificent instrument. In a 

 companion volume to this, The Heavens, numerous examples of 

 stellar clusters and nebulae, observed mostly at Parsonstown with 

 the large telescope which is represented in Plate X. are given. 



We have now come to Gregory's form of telescope. At the 

 principal focus of the larger mirror, placed at the eye-end of the tube, a 

 reversed image of the celestial object AB is formed. In front of the 

 great mirror and on the same axis, a small concave mirror M with 

 its reflecting surface turned towards the larger mirror is arranged. 

 The real image is formed by the larger mirror in front of this 

 small one, which then forms a second and enlarged image doubly 

 reversed, which is magnified by the eye-piece. 



To give an outlet to the pencils of light, the large mirror is 

 pierced with an opening at its centre, near and behind which is 

 fixed the eye-piece tube, so that the observer has the eye turned directly 

 towards the portion of the sky observed, as in the refractor. The 

 light is reduced first by the aperture made in the centre, which 

 diminishes the surface, but particularly by the second reflection on the 

 surface of the small mirror. This is the inconvenience of Gregory's 

 form; the principal advantage of it is the facility with which 

 observations are made, but this does not always dispense with the 

 necessity of a tinder. 



