164 Mr. A. A. Common [May 30, 



discovery with the telescope: his name is, and always will be, 

 associated with the telescope on this account alone. 



Very soon after the introduction of the Galilean telescope, the 

 difficulties that arise from the coloured image produced by a single 

 lens turned attention to the possibility of making a telescope by 

 using the reflecting surface of a concave mirror instead of a lens. 

 Newton, who had imperfectly investigated the decomposition of light 

 produced by its refraction through a prism, was of opinion that the 

 reflecting principle gave the greatest possibilities of increase of power. 

 He invented, and was the first to make, a reflecting telescope on the 

 system that is in use to the present day ; thus the two forms of 

 telescope — the refracting and reflecting — came into use within about 

 60 years of each other. It will be perhaps most convenient in briefly 

 running through the history of the telescope, that I should give what 

 was done in each century. 



Commencing, then, with the first application of the telescope to 

 the investigation of the heavenly bodies by Galileo in 1609, we find 

 that the largest telescope he could make gave only a magnifying 

 power of about 30. 



The first improvement made in the telescope, as left by Galileo, 

 was due to a suggestion — by some attributed to Kepler, but certainly 

 used by Gascoigne— to replace the concave eye-lens that Galileo used 

 by a convex one. Simple as this change looks, it makes an important, 

 indeed vital improvement. The telescope could now be used, by 

 placing a system of lines or a scale in the common focus of the two 

 lenses, to measure the size of the image produced by the large lens ; 

 the axis or line of collimation could be found, and so the telescope 

 could be used on graduated instruments to measure the angular 

 distance of various objects ; in fact, we have now in every essential 

 principle the true astronomical telescope. It is useless as an ordinary 

 telescope, as it inverts the objects looked at, while the Galilean retains 

 them in their natural position. The addition, however, of another 

 lens or pair of lenses reinverts the image, and we then have the 

 ordinary telescope. It was soon found that the single lens surrounds 

 all bright objects with a fringe of colour, always of a width of about 

 one-fiftieth of the diameter of the object-glass, as we must now call 

 the large lens ; and as this width of fringe was the same whatever the 

 focal length of the object-glass, the advantage of increasing this focal 

 length and so getting a larger image without increasing the size of 

 the coloured fringe became apparent, and the telescope therefore was 

 made longer and longer, till a length of over one hundred feet was 

 reached ; in fact, they were made so long that they could not be 

 used. A picture of one of these is shown, from which it can be easily 

 imagined the difficulties of using it must have been very great, yet 

 some most important measurements have been made with these long 

 telescopes. Beyond the suggestions of Gregory and Cassegrain for 

 improvements in the reflecting telescope, little was done with this 

 instrument. 



