94 Things not generally Known. 



pleted his own (Rigaud, On Harriot's Papers, 1833). They were at first 

 called cylinders. The telescopes which Galileo constructed, and others 

 of which he made use for observing Jupiter's satellites, the phases of 

 Venus, and the solar spots, possessed the gradually-increasing powers 

 of magnifying four, seven, and thirty-two linear diameters ; but they 

 never had a higher power. Arago, in the Annuaire for 1842. 



Clock-work is now applied to the equatorial telescope, so as to allow 

 the observer to follow the course of any star, comet, or planet he may 

 wish to observe continuously, without using his hands for the mechani- 

 cal motion of the instrument. 



ANTIQUITY OF TELESCOPES. 



Long tubes were certainly employed by Arabian astrono- 

 mers, and very probably also by the Greeks and Romans ; the 

 exactness of their observations being in some degree attribut- 

 able to their causing the object to be seen through diopters or 

 slits. Abul Hassan speaks very distinctly of tubes, to the ex- 

 tremities of which ocular and object diopters were attached ; 

 and instruments so constructed were used in the observatory 

 founded by Hulagu at Meragha. If stars be more easily dis- 

 covered during twilight by means of tubes, and if a star be 

 sooner revealed to the naked eye through a tube than without 

 it, the reason lies, as Arago has truly observed, in the circum- 

 stance that the tube conceals a great portion of the disturbing 

 light diffused in the atmospheric strata between the star and 

 the eye applied to,,the tube. In like manner, the tube pre- 

 vents the lateral impression of the faint light which the par- 

 ticles of air receive at night from all the other stars in the 

 firmament. The intensity of the image and the size of the 

 star are apparently augmented. Humboldt's Cosmos, vol. iii. 

 p. 53. 



NEWTON'S FIRST REFLECTING TELESCOPE. 



The year 1668 may be regarded as the date of the invention 

 of Newton's Reflecting Telescope. Five years previously, James 

 Gregory had described the manner of constructing a reflecting 

 telescope with two concave specula ; but Newton perceived the 

 disadvantages to be so great, that, according to his statement, 

 he "found it necessary, before attempting any thing in the 

 practice, to alter the design, and place the eye-glass at the side 

 of the tube rather than at the middle." On this improved 

 principle Newton constructed his telescope, which was exa- 

 mined by Charles II. ; it was presented to the Royal Society 

 near the end of 1671, and is carefully preserved by that distin- 

 guished body, with the inscription : 

 "THE FIRST REFLECTING TELESCOPE ; INVENTED BY SIR ISAAC NEWTON, 



AND MADE WITH HIS OWN HANDS." 



Sir David Brewster describes this telescope as consisting of 

 a concave metallic speculum, the radius of curvature of which 



